マルチ言語対応。MCP設定画面の言語切り替え表示にバグあり

23 files changed, 733 insertions, 98 deletions

diff --git a/README.md b/README.md
index aef7708..e259fb4 100644
--- a/README.md
+++ b/README.md
@@ -77,6 +77,11 @@ ollama-web
 | `OLLAMA_WEB_ALLOWED_ORIGINS` | 未設定 | CORS 許可オリジンのカンマ区切り |
 | `OLLAMA_WEB_MCP_STDIO_ALLOWLIST` | 未設定 | stdio MCP で起動を許可する実行ファイル絶対パスのカンマ区切り |
 | `OLLAMA_WEB_MCP_HTTPS_ALLOWLIST` | 未設定 | remote HTTPS MCP 接続を許可するホスト名のカンマ区切り |
+| `OLLAMA_WEB_LANGUAGE` | `ja` | UI と LLM プロンプトの表示言語（`ja` または `en`） |
+
+### 多言語対応
+
+UI 文言と LLM プロンプトは、それぞれ `src/ollama_web/i18n/messages/` と `src/ollama_web/prompts/` の JSON ファイルで管理されています。既定は日本語（`ja`）です。新しい言語を追加する場合は、同じディレクトリに `{lang}.json` を作成し、`OLLAMA_WEB_LANGUAGE` をその言語コードに設定してください。
 
 ### MCP サーバー設定
 
diff --git a/src/ollama_web/app.py b/src/ollama_web/app.py
index d0b85d4..c7477c8 100644
--- a/src/ollama_web/app.py
+++ b/src/ollama_web/app.py
@@ -4,10 +4,13 @@ from __future__ import annotations
 
 import logging
 from pathlib import Path
+from typing import Any
 
 from starlette.applications import Starlette
 from starlette.middleware import Middleware
 from starlette.middleware.cors import CORSMiddleware
+from starlette.requests import Request
+from starlette.responses import Response
 from starlette.routing import Mount, Route
 from starlette.staticfiles import StaticFiles
 
@@ -22,6 +25,19 @@ from .routes import pages as pages_route
 from .routes import sessions as sessions_route
 from .sessions import SessionStore
 
+
+class NoCacheStaticFiles(StaticFiles):
+    """StaticFiles variant that disables browser caching for local development.
+
+    This ensures language changes and JS updates are reflected immediately after
+    a server restart, without requiring users to clear the browser cache.
+    """
+
+    async def get_response(self, path: str, scope: Any) -> Response:
+        response = await super().get_response(path, scope)
+        response.headers["Cache-Control"] = "no-store, no-cache, must-revalidate, max-age=0"
+        return response
+
 _STATIC_DIR = Path(__file__).resolve().parent / "static"
 
 
@@ -97,7 +113,7 @@ def create_app() -> Starlette:
             methods=["PUT"],
             name="mcp_servers_update",
         ),
-        Mount("/static", app=StaticFiles(directory=str(_STATIC_DIR)), name="static"),
+        Mount("/static", app=NoCacheStaticFiles(directory=str(_STATIC_DIR)), name="static"),
     ]
 
     middleware = [Middleware(SecurityHeadersMiddleware), Middleware(AuthMiddleware)]
diff --git a/src/ollama_web/config.py b/src/ollama_web/config.py
index ac567a2..273e8bb 100644
--- a/src/ollama_web/config.py
+++ b/src/ollama_web/config.py
@@ -56,6 +56,9 @@ class Settings:
     max_attachment_text_chars: int = 20000
     max_image_pixels: int = 16_000_000
     max_tool_result_chars: int = 3000
+    language: str = field(
+        default_factory=lambda: os.environ.get("OLLAMA_WEB_LANGUAGE", "ja")
+    )
 
 
 settings = Settings()
diff --git a/src/ollama_web/i18n/__init__.py b/src/ollama_web/i18n/__init__.py
new file mode 100644
index 0000000..56dd217
--- /dev/null
+++ b/src/ollama_web/i18n/__init__.py
@@ -0,0 +1,84 @@
+"""Internationalization support for ollama-web.
+
+Messages live in JSON files under ``i18n/messages/``. The default language is
+read from ``config.settings.language`` and can be overridden per-call via the
+``lang`` argument.
+"""
+
+from __future__ import annotations
+
+import json
+from pathlib import Path
+from typing import Any
+
+from ollama_web.config import settings
+
+_DIR = Path(__file__).resolve().parent
+_MESSAGES_DIR = _DIR / "messages"
+
+
+def _load_messages(lang: str) -> dict[str, Any]:
+    """Load messages for a language, falling back to an empty dict on error."""
+    path = _MESSAGES_DIR / f"{lang}.json"
+    try:
+        with path.open("r", encoding="utf-8") as f:
+            return json.load(f)
+    except (FileNotFoundError, json.JSONDecodeError):
+        return {}
+
+
+_MESSAGES_CACHE: dict[str, dict[str, Any]] = {}
+
+
+def get_messages(lang: str | None = None) -> dict[str, Any]:
+    """Return the full message dictionary for the requested language."""
+    lang = resolve_language(lang)
+    if lang not in _MESSAGES_CACHE:
+        _MESSAGES_CACHE[lang] = _load_messages(lang)
+    return _MESSAGES_CACHE[lang]
+
+
+def _resolve(key: str, data: dict[str, Any]) -> Any:
+    """Resolve a dot-separated key in a nested dictionary."""
+    current: Any = data
+    for part in key.split("."):
+        if not isinstance(current, dict) or part not in current:
+            return None
+        current = current[part]
+    return current
+
+
+def resolve_language(lang_source: str | None = None) -> str:
+    """Resolve the effective language.
+
+    Priority:
+        1. Explicit ``lang_source`` argument if it is a supported filename.
+        2. ``settings.language`` if it is supported.
+        3. ``"ja"`` as a safe default.
+
+    Supported languages are determined by the JSON files present in
+    ``i18n/messages/``.
+    """
+    supported = {p.stem for p in _MESSAGES_DIR.glob("*.json")}
+    candidates = [lang_source, settings.language]
+    for candidate in candidates:
+        if candidate and candidate in supported:
+            return candidate
+    return "ja"
+
+
+def t(key: str, lang: str | None = None, fallback: str | None = None) -> str:
+    """Return a localized message string.
+
+    Args:
+        key: Dot-separated path, e.g. ``"chat.input_placeholder"``.
+        lang: Optional language override. Uses ``settings.language`` when omitted.
+        fallback: Value returned when the key is missing. Defaults to the key itself.
+
+    Returns:
+        The localized string, or the fallback value if missing.
+    """
+    value = _resolve(key, get_messages(resolve_language(lang)))
+    if isinstance(value, str):
+        return value
+    return fallback if fallback is not None else key
diff --git a/src/ollama_web/i18n/messages/en.json b/src/ollama_web/i18n/messages/en.json
new file mode 100644
index 0000000..2195c6e
--- /dev/null
+++ b/src/ollama_web/i18n/messages/en.json
@@ -0,0 +1,100 @@
+{
+  "common": {
+    "send": "Send",
+    "stop": "Stop",
+    "clear": "Clear",
+    "save": "Save",
+    "delete": "Delete"
+  },
+  "chat": {
+    "sessions": "Sessions",
+    "new_session": "New session",
+    "session_list": "Session list",
+    "mcp_settings": "MCP Settings",
+    "model_label": "Model:",
+    "attach_file": "Attach file",
+    "input_placeholder": "Type a message… (Enter to send / Shift+Enter for newline)",
+    "no_sessions": "No sessions",
+    "delete_session": "Delete session",
+    "create_failed": "Failed to create session",
+    "delete_confirm": "Delete this session?",
+    "delete_failed": "Failed to delete session",
+    "upload_failed": "Failed to upload files",
+    "attachment_label": "Attached: {names}",
+    "attachment_files": "Attached files: {names}",
+    "stopped": "Generation stopped",
+    "error_prefix": "Error: {message}",
+    "generating": "ollama is generating a response…",
+    "processing": "Processing…",
+    "no_response": "(No response)",
+    "clear_failed": "Failed to clear",
+    "clear_messages_failed": "Failed to clear messages"
+  },
+  "mcp": {
+    "server_settings": "MCP Server Settings",
+    "add_server": "＋ Add server",
+    "server_name": "Server name",
+    "required": "(required)",
+    "optional": "(optional)",
+    "command": "Command",
+    "args": "Arguments",
+    "env": "Environment variables",
+    "cwd": "Working directory",
+    "url": "URL",
+    "headers": "Headers",
+    "timeout": "Timeout",
+    "sse": "SSE",
+    "stdio": "stdio",
+    "http_stream": "HTTP Stream",
+    "examples": {
+      "command": "e.g. npx",
+      "args": "e.g. -y @modelcontextprotocol/server-filesystem /tmp",
+      "env": "e.g. API_KEY=sk-xxx",
+      "cwd": "e.g. C:\\Users\\Home",
+      "url": "e.g. http://127.0.0.1:9000/mcp",
+      "headers": "e.g. Authorization=Bearer xxx",
+      "timeout": "e.g. 30"
+    }
+  },
+  "login": {
+    "title": "Login",
+    "pin_invalid": "Invalid PIN"
+  },
+  "roles": {
+    "system": "System",
+    "user": "You",
+    "assistant": "Assistant"
+  },
+  "status": {
+    "sending": "Sending…",
+    "fetching_response": "Fetching response…",
+    "done": "Done",
+    "connecting": "Connecting…",
+    "disconnected": "Disconnected",
+    "tool_limit_reached": "Tool call limit reached. Summarizing findings…",
+    "thinking": "Thinking process",
+    "executing_tool": "Executing {label}…",
+    "running": "Running…"
+  },
+  "errors": {
+    "generic": "An error occurred",
+    "vision_model_required": "Select a vision-capable model when sending messages containing images."
+  },
+  "session": {
+    "default_title": "New session"
+  },
+  "html": {
+    "lang": "en"
+  },
+  "tools": {
+    "web_search": "Web search",
+    "scrape_url": "Scraping",
+    "search_and_fetch": "File search and fetch",
+    "pdf_to_text": "PDF text extraction"
+  },
+  "mcp_messages": {
+    "save_failed": "Failed to save: {error}",
+    "saved": "MCP settings saved",
+    "save_error": "Failed to save MCP settings"
+  }
+}
diff --git a/src/ollama_web/i18n/messages/ja.json b/src/ollama_web/i18n/messages/ja.json
new file mode 100644
index 0000000..9d42f85
--- /dev/null
+++ b/src/ollama_web/i18n/messages/ja.json
@@ -0,0 +1,100 @@
+{
+  "common": {
+    "send": "送信",
+    "stop": "停止",
+    "clear": "クリア",
+    "save": "保存",
+    "delete": "削除"
+  },
+  "chat": {
+    "sessions": "セッション",
+    "new_session": "新規セッション",
+    "session_list": "セッション一覧",
+    "mcp_settings": "MCP設定",
+    "model_label": "モデル:",
+    "attach_file": "ファイルを添付",
+    "input_placeholder": "メッセージを入力… (Enter で送信 / Shift+Enter で改行)",
+    "no_sessions": "セッションがありません",
+    "delete_session": "セッションを削除",
+    "create_failed": "セッションの作成に失敗しました",
+    "delete_confirm": "このセッションを削除しますか？",
+    "delete_failed": "セッションの削除に失敗しました",
+    "upload_failed": "ファイルのアップロードに失敗しました",
+    "attachment_label": "添付: {names}",
+    "attachment_files": "添付ファイル: {names}",
+    "stopped": "生成を停止しました",
+    "error_prefix": "エラー: {message}",
+    "generating": "ollama が回答を生成中…",
+    "processing": "処理中…",
+    "no_response": "（応答なし）",
+    "clear_failed": "クリアに失敗しました",
+    "clear_messages_failed": "メッセージのクリアに失敗しました"
+  },
+  "mcp": {
+    "server_settings": "MCPサーバー設定",
+    "add_server": "＋ サーバー追加",
+    "server_name": "サーバー名",
+    "required": "(必須)",
+    "optional": "(任意)",
+    "command": "コマンド",
+    "args": "引数",
+    "env": "環境変数",
+    "cwd": "作業ディレクトリ",
+    "url": "URL",
+    "headers": "ヘッダー",
+    "timeout": "タイムアウト",
+    "sse": "SSE",
+    "stdio": "stdio",
+    "http_stream": "HTTP Stream",
+    "examples": {
+      "command": "例: npx",
+      "args": "例: -y @modelcontextprotocol/server-filesystem /tmp",
+      "env": "例: API_KEY=sk-xxx",
+      "cwd": "例: C:\\Users\\Home",
+      "url": "例: http://127.0.0.1:9000/mcp",
+      "headers": "例: Authorization=Bearer xxx",
+      "timeout": "例: 30"
+    }
+  },
+  "login": {
+    "title": "ログイン",
+    "pin_invalid": "PINが違います"
+  },
+  "roles": {
+    "system": "システム",
+    "user": "あなた",
+    "assistant": "アシスタント"
+  },
+  "status": {
+    "sending": "送信中...",
+    "fetching_response": "応答を取得中...",
+    "done": "完了",
+    "connecting": "接続中...",
+    "disconnected": "切断しました",
+    "tool_limit_reached": "ツール呼び出し上限に達しました。これまでの調査結果を整理中…",
+    "thinking": "思考プロセス",
+    "executing_tool": "{label} を実行中…",
+    "running": "実行中…"
+  },
+  "errors": {
+    "generic": "エラーが発生しました",
+    "vision_model_required": "画像を含むメッセージを送信する場合、vision 対応モデルを選択してください。"
+  },
+  "session": {
+    "default_title": "新しいセッション"
+  },
+  "html": {
+    "lang": "ja"
+  },
+  "tools": {
+    "web_search": "Web検索",
+    "scrape_url": "スクレイピング",
+    "search_and_fetch": "ファイル検索・取得",
+    "pdf_to_text": "PDFテキスト変換"
+  },
+  "mcp_messages": {
+    "save_failed": "保存に失敗しました: {error}",
+    "saved": "MCP設定を保存しました",
+    "save_error": "MCP設定の保存に失敗しました"
+  }
+}
diff --git a/src/ollama_web/llm.py b/src/ollama_web/llm.py
index 693cfa2..80e0ba6 100644
--- a/src/ollama_web/llm.py
+++ b/src/ollama_web/llm.py
@@ -12,7 +12,9 @@ from ollama import Message
 
 
 from .config import settings
+from .i18n import t
 from .mcp import redact_secrets
+from .prompts import get_prompt
 from .tools.registry import ToolRegistry, default_registry
 
 # Maximum successive tool-call rounds before we force a final answer.
@@ -290,15 +292,12 @@ def chat_with_tools(
     )
     yield {
         "type": "status",
-        "message": "ツール呼び出し上限に達しました。これまでの調査結果を整理中…",
+        "message": t("status.tool_limit_reached"),
     }
     messages.append(
         Message(
             role="user",
-            content=(
-                "ツール呼び出しの上限に達しました。これまでの検索結果と会話を元に、"
-                "ユーザーが求めた回答を最終的にまとめてください。"
-            ),
+            content=get_prompt("tool_limit_instruction"),
         )
     )
     try:
@@ -488,15 +487,12 @@ def stream_chat_with_tools(
     )
     yield {
         "type": "status",
-        "message": "ツール呼び出し上限に達しました。これまでの調査結果を整理中…",
+        "message": t("status.tool_limit_reached"),
     }
     messages.append(
         Message(
             role="user",
-            content=(
-                "ツール呼び出しの上限に達しました。これまでの検索結果と会話を元に、"
-                "ユーザーが求めた回答を最終的にまとめてください。"
-            ),
+            content=get_prompt("tool_limit_instruction"),
         )
     )
     t0 = time.time()
diff --git a/src/ollama_web/prompts/__init__.py b/src/ollama_web/prompts/__init__.py
new file mode 100644
index 0000000..59bdb48
--- /dev/null
+++ b/src/ollama_web/prompts/__init__.py
@@ -0,0 +1,57 @@
+"""Localized prompt management for LLM prompts.
+
+Prompts live in JSON files under ``prompts/``. The default language is read from
+``config.settings.language`` and can be overridden per-call via the ``lang``
+argument.
+"""
+
+from __future__ import annotations
+
+import json
+from pathlib import Path
+from typing import Any
+
+from ollama_web.config import settings
+
+_DIR = Path(__file__).resolve().parent
+
+_PROMPTS_CACHE: dict[str, dict[str, Any]] = {}
+
+
+def _load_prompts(lang: str) -> dict[str, Any]:
+    """Load prompts for a language, falling back to an empty dict on error."""
+    path = _DIR / f"{lang}.json"
+    try:
+        with path.open("r", encoding="utf-8") as f:
+            return json.load(f)
+    except (FileNotFoundError, json.JSONDecodeError):
+        return {}
+
+
+def get_prompts(lang: str | None = None) -> dict[str, Any]:
+    """Return the full prompt dictionary for the requested language."""
+    lang = lang or settings.language or "ja"
+    if lang not in _PROMPTS_CACHE:
+        _PROMPTS_CACHE[lang] = _load_prompts(lang)
+    return _PROMPTS_CACHE[lang]
+
+
+def get_prompt(key: str, lang: str | None = None, fallback: str | None = None) -> str:
+    """Return a localized LLM prompt string.
+
+    Args:
+        key: Dot-separated path, e.g. ``"tool_system"``.
+        lang: Optional language override. Uses ``settings.language`` when omitted.
+        fallback: Value returned when the key is missing. Defaults to the key itself.
+
+    Returns:
+        The localized prompt string, or the fallback value if missing.
+    """
+    current: Any = get_prompts(lang)
+    for part in key.split("."):
+        if not isinstance(current, dict) or part not in current:
+            return fallback if fallback is not None else key
+        current = current[part]
+    if isinstance(current, str):
+        return current
+    return fallback if fallback is not None else key
diff --git a/src/ollama_web/prompts/en.json b/src/ollama_web/prompts/en.json
new file mode 100644
index 0000000..53af74b
--- /dev/null
+++ b/src/ollama_web/prompts/en.json
@@ -0,0 +1,18 @@
+{
+  "tool_system": "You can use web search, scraping, and file fetch tools. Do not run more searches than necessary in a single response; review the results before conducting the next investigation. Tool results are summarized, so grasp the important points and use them effectively.",
+  "tool_limit_instruction": "The tool call limit has been reached. Based on the search results and conversation so far, please summarize the final answer the user requested.",
+  "attachment_prefix": "The following is the content of files attached by the user:",
+  "fetch": {
+    "candidate": "Candidate",
+    "title": "Title",
+    "status": "Status",
+    "fetch_failed": "Failed to fetch",
+    "type": "Type",
+    "size": "Size",
+    "chars": "Characters",
+    "separator": "---",
+    "no_results": "No {file_type} files found for query: {query}",
+    "empty_query": "Empty query.",
+    "invalid_type": "Invalid file type."
+  }
+}
diff --git a/src/ollama_web/prompts/ja.json b/src/ollama_web/prompts/ja.json
new file mode 100644
index 0000000..40f6c7c
--- /dev/null
+++ b/src/ollama_web/prompts/ja.json
@@ -0,0 +1,18 @@
+{
+  "tool_system": "あなたはWeb検索・スクレイピング・ファイル取得ツールを利用できます。一度の回答で必要以上に多くの検索を同時に行わず、結果を確認してから次の調査を行ってください。各ツール結果は要約されて提供されるため、重要なポイントを把握して活用してください。",
+  "tool_limit_instruction": "ツール呼び出しの上限に達しました。これまでの検索結果と会話を元に、ユーザーが求めた回答を最終的にまとめてください。",
+  "attachment_prefix": "以下はユーザーが添付したファイルの内容です：",
+  "fetch": {
+    "candidate": "候補",
+    "title": "タイトル",
+    "status": "状態",
+    "fetch_failed": "取得失敗",
+    "type": "形式",
+    "size": "サイズ",
+    "chars": "文字数",
+    "separator": "---",
+    "no_results": "No {file_type} files found for query: {query}",
+    "empty_query": "Empty query.",
+    "invalid_type": "Invalid file type."
+  }
+}
diff --git a/src/ollama_web/routes/chat.py b/src/ollama_web/routes/chat.py
index 2d855b1..30794be 100644
--- a/src/ollama_web/routes/chat.py
+++ b/src/ollama_web/routes/chat.py
@@ -16,7 +16,9 @@ from starlette.requests import Request
 from starlette.responses import StreamingResponse
 
 from ..config import settings
+from ..i18n import t
 from ..mcp import collect_mcp_tools, make_mcp_executor
+from ..prompts import get_prompt
 from ..sessions import SessionStore, is_valid_id
 from ..tools.fetch import pop_fetched_files
 from ..tools.helper.image import resize_image
@@ -27,11 +29,7 @@ logger = logging.getLogger("ollama_web.chat")
 
 # System prompt injected when tools are enabled. Encourages the model to use
 # tools iteratively rather than firing many searches at once.
-_TOOL_SYSTEM_PROMPT = (
-    "あなたはWeb検索・スクレイピング・ファイル取得ツールを利用できます。"
-    "一度の回答で必要以上に多くの検索を同時に行わず、結果を確認してから次の調査を行ってください。"
-    "各ツール結果は要約されて提供されるため、重要なポイントを把握して活用してください。"
-)
+_TOOL_SYSTEM_PROMPT = get_prompt("tool_system")
 
 
 def _limit_text(text: str, limit: int) -> str:
@@ -111,7 +109,7 @@ def _attachment_context(store: SessionStore, session_id: str, file_ids: list[str
         )
     if not parts:
         return ""
-    return "\n\n".join(["以下はユーザーが添付したファイルの内容です：", *parts])
+    return "\n\n".join([get_prompt("attachment_prefix"), *parts])
 
 
 def _collect_images(store: SessionStore, session_id: str, file_ids: list[str]) -> list[bytes]:
@@ -257,7 +255,7 @@ async def _chat_event_stream(
         # and re-renders in the chat pane.
         assistant_text = (
             f"[ERROR] {error_msg}\n\n"
-            "画像を含むメッセージを送信する場合、vision 対応モデルを選択してください。"
+            f"{t('errors.vision_model_required')}"
         )
 
     finally:
diff --git a/src/ollama_web/routes/pages.py b/src/ollama_web/routes/pages.py
index 52af9e8..45a3a01 100644
--- a/src/ollama_web/routes/pages.py
+++ b/src/ollama_web/routes/pages.py
@@ -9,9 +9,11 @@ from starlette.responses import Response
 from starlette.templating import Jinja2Templates
 
 from ollama_web import llm
+from ollama_web.i18n import get_messages, t
 
 _TEMPLATES_DIR = Path(__file__).resolve().parent.parent / "templates"
 templates = Jinja2Templates(directory=str(_TEMPLATES_DIR))
+templates.env.globals["t"] = t
 
 
 async def index(request: Request) -> Response:
@@ -22,8 +24,13 @@ async def index(request: Request) -> Response:
     return templates.TemplateResponse(
         request,
         "index.html",
-        {"models": models, "default_model": default_model},
+        {
+            "models": models,
+            "default_model": default_model,
+            "messages": get_messages(),
+        },
         media_type="text/html; charset=utf-8",
+        headers={"Cache-Control": "no-store, no-cache, must-revalidate, max-age=0"},
     )
 
 
@@ -32,6 +39,9 @@ async def login(request: Request) -> Response:
     return templates.TemplateResponse(
         request,
         "login.html",
-        {},
+        {
+            "messages": get_messages(),
+        },
         media_type="text/html; charset=utf-8",
+        headers={"Cache-Control": "no-store, no-cache, must-revalidate, max-age=0"},
     )
diff --git a/src/ollama_web/sessions.py b/src/ollama_web/sessions.py
index 645f934..25f345b 100644
--- a/src/ollama_web/sessions.py
+++ b/src/ollama_web/sessions.py
@@ -12,6 +12,7 @@ from pathlib import Path
 from typing import Any, cast
 
 from .config import settings
+from .i18n import t
 
 _UUID_HEX_RE = re.compile(r"^[0-9a-f]{32}$")
 
@@ -156,7 +157,7 @@ class SessionStore:
         now = _now()
         session = {
             "id": session_id,
-            "title": title or "新しいセッション",
+            "title": title or t("session.default_title"),
             "created_at": now,
             "updated_at": now,
             "messages": [],
diff --git a/src/ollama_web/static/app.js b/src/ollama_web/static/app.js
index 00269b3..492f9e4 100644
--- a/src/ollama_web/static/app.js
+++ b/src/ollama_web/static/app.js
@@ -170,7 +170,7 @@ function addMessage(role) {
   div.className = `message ${role}`;
   const roleEl = document.createElement("div");
   roleEl.className = "role";
-  roleEl.textContent = role === "user" ? "あなた" : "アシスタント";
+  roleEl.textContent = role === "user" ? t("roles.user") : t("roles.assistant");
   const body = document.createElement("div");
   body.className = "body";
   div.append(roleEl, body);
@@ -190,7 +190,7 @@ function addToolBubble(parentExtras, name, args) {
   det.className = "tool";
   const label = typeof window.toolLabel === "function" ? window.toolLabel(name) : name;
   const sum = document.createElement("summary");
-  sum.textContent = `${label} を実行中…`;
+  sum.textContent = t("status.executing_tool", "Executing {label}…").replace("{label}", label);
   det.appendChild(sum);
   if (args && Object.keys(args).length) {
     const a = document.createElement("div");
@@ -210,7 +210,7 @@ function addThinkingBubble(parentExtras) {
   const det = document.createElement("details");
   det.className = "thinking";
   const sum = document.createElement("summary");
-  sum.textContent = "思考プロセス";
+  sum.textContent = t("status.thinking");
   const body = document.createElement("div");
   det.append(sum, body);
   parentExtras.appendChild(det);
@@ -261,7 +261,7 @@ function renderSessionList(sessions) {
   if (!sessions.length) {
     const li = document.createElement("li");
     li.className = "empty";
-    li.textContent = "セッションがありません";
+    li.textContent = t("chat.no_sessions");
     sessionListEl.appendChild(li);
     return;
   }
@@ -274,8 +274,8 @@ function renderSessionList(sessions) {
     title.textContent = s.title || s.id;
     const del = document.createElement("button");
     del.className = "delete";
-    del.textContent = "削除";
-    del.title = "セッションを削除";
+    del.textContent = t("common.delete");
+    del.title = t("chat.delete_session");
     del.addEventListener("click", (e) => {
       e.stopPropagation();
       deleteSession(s.id);
@@ -295,12 +295,12 @@ async function createSession() {
     await selectSession(session.id);
   } catch (err) {
     console.error("failed to create session", err);
-    alert("セッションの作成に失敗しました");
+    alert(t("chat.create_failed"));
   }
 }
 
 async function deleteSession(id) {
-  if (!confirm("このセッションを削除しますか？")) return;
+  if (!confirm(t("chat.delete_confirm"))) return;
   try {
     const res = await csrfFetch(`/api/sessions/${id}`, { method: "DELETE" });
     if (!res.ok) throw new Error(res.statusText);
@@ -314,7 +314,7 @@ async function deleteSession(id) {
     await loadSessions();
   } catch (err) {
     console.error("failed to delete session", err);
-    alert("セッションの削除に失敗しました");
+    alert(t("chat.delete_failed"));
   }
 }
 
@@ -381,7 +381,7 @@ function renderSession() {
             return f ? f.name : fid;
           })
           .join(", ");
-        info.textContent = `添付: ${names}`;
+        info.textContent = t("chat.attachment_label", "Attached: {names}").replace("{names}", names);
         body.appendChild(info);
       }
     } else if (m.role === "assistant") {
@@ -437,7 +437,7 @@ async function uploadFiles(files) {
     await selectSession(currentSessionId);
   } catch (err) {
     console.error("failed to upload files", err);
-    alert("ファイルのアップロードに失敗しました");
+    alert(t("chat.upload_failed"));
   }
 }
 
@@ -489,7 +489,7 @@ async function send() {
   let displayContent = text;
   if (pendingFiles.length) {
     const names = pendingFiles.map((f) => f.name).join(", ");
-    displayContent += (text ? "\n\n" : "") + `添付ファイル: ${names}`;
+    displayContent += (text ? "\n\n" : "") + t("chat.attachment_files", "Attached files: {names}").replace("{names}", names);
   }
   userBody.innerHTML = renderMarkdown(displayContent);
   highlightIn(userBody);
@@ -527,7 +527,9 @@ async function send() {
     });
   } catch (err) {
     assistantBody.textContent =
-      err.name === "AbortError" ? "生成を停止しました" : `エラー: ${err.message}`;
+      err.name === "AbortError"
+        ? t("chat.stopped")
+        : t("chat.error_prefix", "Error: {message}").replace("{message}", err.message);
     abortController = null;
     cancelBtn.disabled = true;
     sendBtn.disabled = false;
@@ -535,7 +537,7 @@ async function send() {
   }
 
   if (!res.ok || !res.body) {
-    assistantBody.textContent = `エラー: ${res.status}`;
+    assistantBody.textContent = t("chat.error_prefix", "Error: {message}").replace("{message}", res.status);
     abortController = null;
     cancelBtn.disabled = true;
     sendBtn.disabled = false;
@@ -606,7 +608,10 @@ async function send() {
       } else if (ev.type === "tool_end") {
         const b = pendingTools.shift();
         if (b) {
-          b.sum.textContent = b.sum.textContent.replace("実行中…", "完了");
+          b.sum.textContent = b.sum.textContent.replace(
+            t("status.running", "Running…"),
+            t("status.done")
+          );
           b.result.textContent = (ev.result || "").slice(0, 2000);
           chatEl.scrollTop = chatEl.scrollHeight;
         }
@@ -621,7 +626,7 @@ async function send() {
           waiting.className = "waiting-msg";
           waiting.style.color = "var(--muted)";
           waiting.style.fontStyle = "italic";
-          waiting.textContent = "ollama が回答を生成中…";
+          waiting.textContent = t("chat.generating");
           assistantBody.appendChild(waiting);
         }
       } else if (ev.type === "status") {
@@ -634,7 +639,7 @@ async function send() {
           waiting.className = "waiting-msg";
           waiting.style.color = "var(--muted)";
           waiting.style.fontStyle = "italic";
-          waiting.textContent = ev.message || "処理中…";
+          waiting.textContent = ev.message || t("chat.processing");
           assistantBody.appendChild(waiting);
         }
       } else if (ev.type === "thinking") {
@@ -651,7 +656,7 @@ async function send() {
         }
         const span = document.createElement("span");
         span.style.color = "#f85149";
-        span.textContent = `エラー: ${ev.message}`;
+        span.textContent = t("chat.error_prefix", "Error: {message}").replace("{message}", ev.message);
         assistantBody.appendChild(span);
       } else if (ev.type === "done") {
         break;
@@ -688,7 +693,7 @@ async function send() {
   }
 
   if (!assistantText && !pendingTools.length && !thinkingBody && !gotError) {
-    assistantBody.textContent = "（応答なし）";
+    assistantBody.textContent = t("chat.no_response");
   }
 
   abortController = null;
@@ -704,7 +709,7 @@ async function send() {
 
   if (errorHtml && chatEl.lastElementChild) {
     const lastBody = chatEl.lastElementChild.querySelector(".body");
-    if (lastBody && lastBody.textContent.trim() === "（応答なし）") {
+    if (lastBody && lastBody.textContent.trim() === t("chat.no_response")) {
       lastBody.innerHTML = errorHtml;
     }
   }
@@ -736,7 +741,7 @@ async function clearMessages() {
     });
     if (!res.ok) throw new Error(res.statusText);
     const data = await res.json();
-    if (!data.cleared) throw new Error("クリアに失敗しました");
+    if (!data.cleared) throw new Error(t("chat.clear_failed"));
 
     if (currentSession) {
       currentSession.messages = [];
@@ -747,7 +752,7 @@ async function clearMessages() {
     renderAttachments();
   } catch (err) {
     console.error("failed to clear messages", err);
-    alert("メッセージのクリアに失敗しました");
+    alert(t("chat.clear_messages_failed"));
   }
 }
 
diff --git a/src/ollama_web/static/i18n.js b/src/ollama_web/static/i18n.js
new file mode 100644
index 0000000..985b452
--- /dev/null
+++ b/src/ollama_web/static/i18n.js
@@ -0,0 +1,28 @@
+/** Small i18n helper for the browser.
+ *
+ * The server injects ``window.I18N`` as a nested message object. This module
+ * exposes ``t(key, fallback)`` to resolve dot-separated keys safely.
+ */
+(function () {
+  "use strict";
+
+  function t(key, fallback) {
+    if (typeof key !== "string") {
+      return fallback !== undefined ? fallback : String(key);
+    }
+    const parts = key.split(".");
+    let current = window.I18N;
+    for (const part of parts) {
+      if (current == null || typeof current !== "object" || !(part in current)) {
+        return fallback !== undefined ? fallback : key;
+      }
+      current = current[part];
+    }
+    if (typeof current === "string") {
+      return current;
+    }
+    return fallback !== undefined ? fallback : key;
+  }
+
+  window.t = t;
+})();
diff --git a/src/ollama_web/static/login.js b/src/ollama_web/static/login.js
index 1f91868..4c9151b 100644
--- a/src/ollama_web/static/login.js
+++ b/src/ollama_web/static/login.js
@@ -14,7 +14,7 @@ form.addEventListener("submit", async (event) => {
     body: JSON.stringify({ pin: pinInput.value }),
   });
   if (!res.ok) {
-    errorEl.textContent = "PINが違います";
+    errorEl.textContent = t("login.pin_invalid", "Invalid PIN");
     return;
   }
   const requestedNext = new URLSearchParams(window.location.search).get("next") || "/";
diff --git a/src/ollama_web/static/mcp.js b/src/ollama_web/static/mcp.js
index ff3a27b..dc36b03 100644
--- a/src/ollama_web/static/mcp.js
+++ b/src/ollama_web/static/mcp.js
@@ -51,13 +51,13 @@
     const header = document.createElement("div");
     header.className = "mcp-server-header";
 
-    const nameInput = createInput(name, "サーバー名", "text");
+    const nameInput = createInput(name, t("mcp.server_name"), "text");
     nameInput.className = "mcp-server-name";
 
     const delBtn = document.createElement("button");
     delBtn.type = "button";
     delBtn.className = "mcp-server-delete";
-    delBtn.textContent = "削除";
+    delBtn.textContent = t("common.delete");
     delBtn.addEventListener("click", () => card.remove());
 
     header.append(nameInput, delBtn);
@@ -65,8 +65,8 @@
     const typeSelect = document.createElement("select");
     typeSelect.className = "mcp-server-type";
     typeSelect.innerHTML = `
-      <option value="stdio">stdio</option>
-      <option value="http">HTTP Stream</option>
+      <option value="stdio">${t("mcp.stdio", "stdio")}</option>
+      <option value="http">${t("mcp.http_stream", "HTTP Stream")}</option>
     `;
     const isHttp = "url" in server;
     typeSelect.value = isHttp ? "http" : "stdio";
@@ -78,7 +78,7 @@
       const wrap = document.createElement("label");
       wrap.className = "mcp-field";
       const span = document.createElement("span");
-      span.textContent = `${label} ${required ? "(必須)" : "(任意)"}`;
+      span.textContent = `${label} ${required ? t("mcp.required") : t("mcp.optional")}`;
       wrap.append(span, control);
       return wrap;
     }
@@ -87,11 +87,11 @@
       fieldsEl.innerHTML = "";
       const type = typeSelect.value;
       if (type === "stdio") {
-        const command = createInput(server.command || "", "例: npx", "text");
+        const command = createInput(server.command || "", t("mcp.examples.command"), "text");
         command.dataset.key = "command";
         const args = createTextarea(
           (server.args || []).join("\n"),
-          "例: -y @modelcontextprotocol/server-filesystem /tmp",
+          t("mcp.examples.args"),
           3
         );
         args.dataset.key = "args";
@@ -99,41 +99,41 @@
           Object.entries(server.env || {})
             .map(([k, v]) => `${k}=${v}`)
             .join("\n"),
-          "例: API_KEY=sk-xxx",
+          t("mcp.examples.env"),
           3
         );
         env.dataset.key = "env";
-        const cwd = createInput(server.cwd || "", "例: C:\\Users\\Home", "text");
+        const cwd = createInput(server.cwd || "", t("mcp.examples.cwd"), "text");
         cwd.dataset.key = "cwd";
 
         fieldsEl.append(
-          wrapField("command", command, true),
-          wrapField("args", args, false),
-          wrapField("env", env, false),
-          wrapField("cwd", cwd, false)
+          wrapField(t("mcp.command"), command, true),
+          wrapField(t("mcp.args"), args, false),
+          wrapField(t("mcp.env"), env, false),
+          wrapField(t("mcp.cwd"), cwd, false)
         );
       } else {
-        const url = createInput(server.url || "", "例: http://127.0.0.1:9000/mcp", "text");
+        const url = createInput(server.url || "", t("mcp.examples.url"), "text");
         url.dataset.key = "url";
         const headers = createTextarea(
           Object.entries(server.headers || {})
             .map(([k, v]) => `${k}=${v}`)
             .join("\n"),
-          "例: Authorization=Bearer xxx",
+          t("mcp.examples.headers"),
           3
         );
         headers.dataset.key = "headers";
         const timeout = createInput(
           server.timeout !== undefined ? String(server.timeout) : "",
-          "例: 30",
+          t("mcp.examples.timeout"),
           "number"
         );
         timeout.dataset.key = "timeout";
 
         fieldsEl.append(
-          wrapField("url", url, true),
-          wrapField("headers", headers, false),
-          wrapField("timeout", timeout, false)
+          wrapField(t("mcp.url"), url, true),
+          wrapField(t("mcp.headers"), headers, false),
+          wrapField(t("mcp.timeout"), timeout, false)
         );
       }
     }
@@ -247,15 +247,16 @@
       });
       if (!res.ok) {
         const data = await res.json().catch(() => ({}));
-        alert(`保存に失敗しました: ${(data.error || []).join(" / ") || res.statusText}`);
+        const errorText = (data.error || []).join(" / ") || res.statusText;
+        alert(t("mcp_messages.save_failed", "Failed to save: {error}").replace("{error}", errorText));
         return;
       }
       currentConfig = await res.json();
-      alert("MCP設定を保存しました");
+      alert(t("mcp_messages.saved"));
       closeModal();
     } catch (err) {
       console.error("failed to save MCP config", err);
-      alert("MCP設定の保存に失敗しました");
+      alert(t("mcp_messages.save_error"));
     }
   });
 
diff --git a/src/ollama_web/static/tools.js b/src/ollama_web/static/tools.js
index 59d5c02..ab392ca 100644
--- a/src/ollama_web/static/tools.js
+++ b/src/ollama_web/static/tools.js
@@ -1,15 +1,17 @@
 "use strict";
 
 window.TOOLS = [
-  { name: "web_search", label: "Web検索" },
-  { name: "scrape_url", label: "スクレイピング" },
-  { name: "search_and_fetch", label: "ファイル検索・取得" },
-  { name: "pdf_to_text", label: "PDFテキスト変換" },
+  { name: "web_search", i18n_key: "tools.web_search" },
+  { name: "scrape_url", i18n_key: "tools.scrape_url" },
+  { name: "search_and_fetch", i18n_key: "tools.search_and_fetch" },
+  { name: "pdf_to_text", i18n_key: "tools.pdf_to_text" },
 ];
 
 window.toolLabel = function toolLabel(name) {
   const builtin = window.TOOLS.find((t) => t.name === name);
-  if (builtin) return builtin.label;
+  if (builtin) {
+    return typeof window.t === "function" ? window.t(builtin.i18n_key, builtin.name) : builtin.name;
+  }
 
   const mcpMatch = /^mcp__([^_]+)__(.+)$/.exec(name);
   if (mcpMatch) {
diff --git a/src/ollama_web/templates/index.html b/src/ollama_web/templates/index.html
index f59066d..11457e8 100644
--- a/src/ollama_web/templates/index.html
+++ b/src/ollama_web/templates/index.html
@@ -1,5 +1,5 @@
 <!DOCTYPE html>
-<html lang="ja">
+<html lang="{{ t('html.lang') }}">
 <head>
   <meta charset="UTF-8" />
   <meta name="viewport" content="width=device-width, initial-scale=1.0" />
@@ -11,26 +11,30 @@
   <script src="{{ url_for('static', path='vendor/purify.min.js') }}"></script>
   <script src="{{ url_for('static', path='vendor/highlight.min.js') }}"></script>
   <script src="{{ url_for('static', path='vendor/katex.min.js') }}"></script>
+  <script>
+    window.I18N = {{ messages | tojson }};
+  </script>
+  <script src="{{ url_for('static', path='i18n.js') }}"></script>
 </head>
 <body>
   <aside id="sidebar">
     <div class="sidebar-header">
-      <h2>セッション</h2>
-      <button id="new-session-btn" type="button" title="新規セッション">＋</button>
+      <h2>{{ t('chat.sessions') }}</h2>
+      <button id="new-session-btn" type="button" title="{{ t('chat.new_session') }}">＋</button>
     </div>
     <ul id="session-list"></ul>
     <div class="sidebar-footer">
-      <button id="mcp-settings-btn" type="button">MCP設定</button>
+      <button id="mcp-settings-btn" type="button">{{ t('chat.mcp_settings') }}</button>
     </div>
   </aside>
   <div id="sidebar-backdrop"></div>
 
   <div id="main">
     <header>
-      <button id="menu-btn" type="button" aria-label="セッション一覧">☰</button>
+      <button id="menu-btn" type="button" aria-label="{{ t('chat.session_list') }}">☰</button>
       <h1>ollama-web</h1>
       <div class="controls">
-        <label for="model-select">モデル:</label>
+        <label for="model-select">{{ t('chat.model_label') }}</label>
         <select id="model-select">
           {% if models %}
             {% for m in models %}
@@ -43,7 +47,7 @@
         <label class="think-toggle">
           <input type="checkbox" id="think-toggle" /> thinking
         </label>
-        <button id="clear-btn" type="button">クリア</button>
+        <button id="clear-btn" type="button">{{ t('common.clear') }}</button>
       </div>
     </header>
 
@@ -53,14 +57,14 @@
 
     <form id="input-form">
       <div class="input-row">
-        <label class="attach-btn" title="ファイルを添付">
+        <label class="attach-btn" title="{{ t('chat.attach_file') }}">
           <input type="file" id="file-input" multiple accept="*/*" />
           📎
         </label>
-        <textarea id="input" placeholder="メッセージを入力… (Enter で送信 / Shift+Enter で改行)"
+        <textarea id="input" placeholder="{{ t('chat.input_placeholder') }}"
                   rows="2" autofocus></textarea>
-        <button type="submit" id="send-btn">送信</button>
-        <button type="button" id="cancel-btn" disabled>停止</button>
+        <button type="submit" id="send-btn">{{ t('common.send') }}</button>
+        <button type="button" id="cancel-btn" disabled>{{ t('common.stop') }}</button>
       </div>
     </form>
   </div>
@@ -68,15 +72,15 @@
   <div id="mcp-modal" class="modal">
     <div class="modal-content">
       <div class="modal-header">
-        <h2>MCPサーバー設定</h2>
+        <h2>{{ t('mcp.server_settings') }}</h2>
         <button id="mcp-modal-close" type="button" class="close-btn">&times;</button>
       </div>
       <div class="modal-body">
         <div id="mcp-servers"></div>
-        <button id="mcp-add-btn" type="button" class="secondary">＋ サーバー追加</button>
+        <button id="mcp-add-btn" type="button" class="secondary">{{ t('mcp.add_server') }}</button>
       </div>
       <div class="modal-footer">
-        <button id="mcp-save-btn" type="button">保存</button>
+        <button id="mcp-save-btn" type="button">{{ t('common.save') }}</button>
       </div>
     </div>
   </div>
diff --git a/src/ollama_web/templates/login.html b/src/ollama_web/templates/login.html
index af88b52..128c842 100644
--- a/src/ollama_web/templates/login.html
+++ b/src/ollama_web/templates/login.html
@@ -1,10 +1,14 @@
 <!DOCTYPE html>
-<html lang="ja">
+<html lang="{{ t('html.lang') }}">
 <head>
   <meta charset="UTF-8" />
   <meta name="viewport" content="width=device-width, initial-scale=1.0" />
   <title>ollama-web login</title>
   <link rel="stylesheet" href="{{ url_for('static', path='style.css') }}" />
+  <script>
+    window.I18N = {{ messages | tojson }};
+  </script>
+  <script src="{{ url_for('static', path='i18n.js') }}"></script>
 </head>
 <body class="login-page">
   <main class="login-shell">
@@ -12,7 +16,7 @@
       <h1>ollama-web</h1>
       <label for="pin-input">PIN</label>
       <input id="pin-input" name="pin" type="password" autocomplete="current-password" autofocus />
-      <button type="submit">ログイン</button>
+      <button type="submit">{{ t('login.title') }}</button>
       <p id="login-error" role="alert"></p>
     </form>
   </main>
diff --git a/src/ollama_web/tools/fetch.py b/src/ollama_web/tools/fetch.py
index e5af37a..20031f1 100644
--- a/src/ollama_web/tools/fetch.py
+++ b/src/ollama_web/tools/fetch.py
@@ -8,6 +8,7 @@ from pathlib import Path
 from typing import Any
 
 from ..config import settings
+from ..prompts import get_prompt, get_prompts
 from .helper.pdf import extract_pdf_text
 from .helper.safe_http import fetch_public_bytes
 from .scrape import scrape_url
@@ -147,11 +148,11 @@ def search_and_fetch(
       successfully fetched file.
     """
     if not query or not query.strip():
-        return "Empty query."
+        return get_prompt("fetch.empty_query")
 
     ft = _normalize_file_type(file_type)
     if not ft:
-        return "Invalid file type."
+        return get_prompt("fetch.invalid_type")
 
     limit = min(
         max_results
@@ -177,7 +178,7 @@ def search_and_fetch(
                 break
 
     if not matched:
-        return f"No {ft} files found for query: {query}"
+        return get_prompt("fetch.no_results", fallback="").replace("{file_type}", ft).replace("{query}", query)
 
     outputs: list[str] = []
     for idx, item in enumerate(matched, 1):
@@ -186,9 +187,10 @@ def search_and_fetch(
         try:
             data = _fetch_bytes(url)
         except Exception as exc:  # noqa: BLE001
+            p = get_prompts()
             outputs.append(
-                f"=== 候補 {idx} ===\n"
-                f"URL: {url}\nタイトル: {title}\n状態: 取得失敗 ({exc})\n"
+                f"=== {p['candidate']} {idx} ===\n"
+                f"URL: {url}\n{p['title']}: {title}\n{p['status']}: {p['fetch_failed']} ({exc})\n"
             )
             continue
 
@@ -199,14 +201,15 @@ def search_and_fetch(
         name = Path(httpx.URL(url).path or f"file.{ft}").name
         if session_id:
             store_fetched_file(session_id, name, data, mime, text, url)
+        p = get_prompts()
         outputs.append(
-            f"=== 候補 {idx} ===\n"
+            f"=== {p['candidate']} {idx} ===\n"
             f"URL: {url}\n"
-            f"タイトル: {title}\n"
-            f"形式: {mime}\n"
-            f"サイズ: {len(data)} bytes\n"
-            f"文字数: {len(text)}\n"
-            f"---\n{text}\n"
+            f"{p['title']}: {title}\n"
+            f"{p['type']}: {mime}\n"
+            f"{p['size']}: {len(data)} bytes\n"
+            f"{p['chars']}: {len(text)}\n"
+            f"{p['separator']}\n{text}\n"
         )
 
     return "\n".join(outputs)
diff --git a/tests/test_i18n.py b/tests/test_i18n.py
new file mode 100644
index 0000000..48f8b9f
--- /dev/null
+++ b/tests/test_i18n.py
@@ -0,0 +1,110 @@
+"""Tests that i18n keys referenced in code and templates exist in JSON files."""
+
+from __future__ import annotations
+
+import json
+import re
+from pathlib import Path
+
+import pytest
+
+from ollama_web import i18n, prompts
+
+
+_SRC = Path(__file__).resolve().parent.parent / "src" / "ollama_web"
+
+
+def _json_keys(data: dict, prefix: str = "") -> set[str]:
+    """Return all dot-separated keys in a nested dict."""
+    keys: set[str] = set()
+    for key, value in data.items():
+        full = f"{prefix}.{key}" if prefix else key
+        keys.add(full)
+        if isinstance(value, dict):
+            keys.update(_json_keys(value, full))
+    return keys
+
+
+def _extract_template_keys(path: Path) -> set[str]:
+    """Find t('...') calls in Jinja2 templates."""
+    text = path.read_text(encoding="utf-8")
+    return set(re.findall(r"{{\s*t\(['\"]([^'\"]+)['\"]", text))
+
+
+def _extract_js_keys(path: Path) -> set[str]:
+    """Find t("...") / t('...') calls in JavaScript files."""
+    text = path.read_text(encoding="utf-8")
+    return set(re.findall(r"\bt\(['\"]([^'\"]+)['\"]", text))
+
+
+def _extract_python_t_keys(path: Path) -> set[str]:
+    """Find t(...) calls in Python source."""
+    text = path.read_text(encoding="utf-8")
+    return set(re.findall(r"\bt\(['\"]([^'\"]+)['\"]", text))
+
+
+def _extract_python_prompt_keys(path: Path) -> set[str]:
+    """Find get_prompt(...) calls in Python source."""
+    text = path.read_text(encoding="utf-8")
+    return set(re.findall(r"\bget_prompt\(['\"]([^'\"]+)['\"]", text))
+
+
+def _referenced_message_keys() -> set[str]:
+    """Collect every message key referenced from templates, JS and Python."""
+    referenced: set[str] = set()
+    for path in _SRC.rglob("*.html"):
+        referenced.update(_extract_template_keys(path))
+    for path in _SRC.rglob("*.js"):
+        referenced.update(_extract_js_keys(path))
+    for path in _SRC.rglob("*.py"):
+        referenced.update(_extract_python_t_keys(path))
+    return referenced
+
+
+def _referenced_prompt_keys() -> set[str]:
+    """Collect every prompt key referenced from Python."""
+    referenced: set[str] = set()
+    for path in _SRC.rglob("*.py"):
+        referenced.update(_extract_python_prompt_keys(path))
+    return referenced
+
+
+@pytest.mark.parametrize("lang", ["ja", "en"])
+def test_all_message_keys_exist(lang: str):
+    """Every referenced message key must exist for each supported language."""
+    messages = i18n.get_messages(lang)
+    available = _json_keys(messages)
+    missing = _referenced_message_keys() - available
+    assert not missing, f"Missing keys in i18n/messages/{lang}.json: {sorted(missing)}"
+
+
+@pytest.mark.parametrize("lang", ["ja", "en"])
+def test_all_prompt_keys_exist(lang: str):
+    """Every referenced prompt key must exist for each supported language."""
+    prompt_data = prompts.get_prompts(lang)
+    available = _json_keys(prompt_data)
+    missing = _referenced_prompt_keys() - available
+    assert not missing, f"Missing keys in prompts/{lang}.json: {sorted(missing)}"
+
+
+@pytest.mark.parametrize("lang", ["ja", "en"])
+def test_json_files_are_valid(lang: str):
+    """Both localization JSON files must be valid and parseable."""
+    assert isinstance(i18n.get_messages(lang), dict)
+    assert isinstance(prompts.get_prompts(lang), dict)
+
+
+def test_t_returns_expected_strings():
+    """Basic sanity checks for the t() helper."""
+    assert i18n.t("common.send") == "送信"
+    assert i18n.t("common.send", lang="en") == "Send"
+    assert i18n.t("missing.key", fallback="fallback") == "fallback"
+    assert i18n.t("missing.key") == "missing.key"
+
+
+def test_get_prompt_returns_expected_strings():
+    """Basic sanity checks for the get_prompt() helper."""
+    assert prompts.get_prompt("tool_system") == prompts.get_prompts("ja")["tool_system"]
+    assert prompts.get_prompt("tool_system", lang="en") == prompts.get_prompts("en")["tool_system"]
+    assert prompts.get_prompt("missing.prompt", fallback="fallback") == "fallback"
+    assert prompts.get_prompt("missing.prompt") == "missing.prompt"
diff --git a/tests/test_i18n_rendering.py b/tests/test_i18n_rendering.py
new file mode 100644
index 0000000..95015d5
--- /dev/null
+++ b/tests/test_i18n_rendering.py
@@ -0,0 +1,72 @@
+"""Tests that page rendering respects the configured language consistently."""
+
+from __future__ import annotations
+
+import pytest
+from starlette.testclient import TestClient
+
+from ollama_web.app import create_app
+from ollama_web.config import settings
+from ollama_web.i18n import get_messages, t
+
+
+@pytest.fixture
+def reset_language():
+    """Reset language to the initial value after each test."""
+    original = settings.language
+    yield
+    settings.language = original
+    if hasattr(get_messages, "cache_clear"):
+        get_messages.cache_clear()
+
+
+def _render_index() -> str:
+    """Render index.html and return the HTML body."""
+    app = create_app()
+    with TestClient(app) as client:
+        response = client.post("/api/auth/login", json={"pin": settings.pin})
+        assert response.status_code == 200
+        response = client.get("/")
+        return response.text
+
+
+def test_english_language_consistency(reset_language):
+    """When settings.language='en', all UI text including MCP must be English."""
+    settings.language = "en"
+    html = _render_index()
+    # HTML-side labels
+    assert 'lang="en"' in html
+    assert "MCP Settings" in html
+    assert "MCP Server Settings" in html
+    assert "＋ Add server" in html
+    # Japanese labels must not appear anywhere
+    assert "MCP設定" not in html
+    assert "MCPサーバー設定" not in html
+    assert "＋ サーバー追加" not in html
+    assert "セッション" not in html
+    assert "モデル:" not in html
+
+
+def test_japanese_language_consistency(reset_language):
+    """When settings.language='ja', all UI text including MCP must be Japanese."""
+    settings.language = "ja"
+    html = _render_index()
+    # HTML-side labels
+    assert 'lang="ja"' in html
+    assert "MCP設定" in html
+    assert "MCPサーバー設定" in html
+    assert "＋ サーバー追加" in html
+    # English labels must not appear anywhere
+    assert "MCP Settings" not in html
+    assert "MCP Server Settings" not in html
+    assert "＋ Add server" not in html
+    assert "Sessions" not in html
+    assert "Model:" not in html
+
+
+def test_t_helper_respects_language_override():
+    """The t() helper must return the requested language string."""
+    assert t("common.send", lang="en") == "Send"
+    assert t("common.send", lang="ja") == "送信"
+    assert t("mcp.server_settings", lang="en") == "MCP Server Settings"
+    assert t("mcp.server_settings", lang="ja") == "MCPサーバー設定"