feat: unify language handling across the application

- Updated the language configuration to use a single source of truth from `DEFAULT_LANGUAGE` for the bot, API, and Mini App, eliminating auto-detection from user settings.
- Refactored the `get_lang` function to always return `DEFAULT_LANGUAGE`, ensuring consistent language usage throughout the application.
- Modified the handling of language in various components, including API responses and UI elements, to reflect the new language management approach.
- Enhanced documentation and comments to clarify the changes in language handling.
- Added unit tests to verify the new language handling behavior and ensure coverage for the updated functionality.

tests/test_api_dependencies.py

@@ -10,24 +10,27 @@ import duty_teller.config as config
 
 
 class TestLangFromAcceptLanguage:
-    """Tests for _lang_from_accept_language."""
+    """Tests for _lang_from_accept_language: always returns config.DEFAULT_LANGUAGE."""
 
-    def test_none_returns_default(self):
+    def test_always_returns_default_language(self):
+        """Header is ignored; result is always config.DEFAULT_LANGUAGE."""
         assert deps._lang_from_accept_language(None) == config.DEFAULT_LANGUAGE
-
-    def test_empty_string_returns_default(self):
         assert deps._lang_from_accept_language("") == config.DEFAULT_LANGUAGE
         assert deps._lang_from_accept_language("   ") == config.DEFAULT_LANGUAGE
+        assert deps._lang_from_accept_language("ru-RU,ru;q=0.9") == config.DEFAULT_LANGUAGE
+        assert deps._lang_from_accept_language("en-US") == config.DEFAULT_LANGUAGE
+        assert deps._lang_from_accept_language("zz") == config.DEFAULT_LANGUAGE
+        assert deps._lang_from_accept_language("x") == config.DEFAULT_LANGUAGE
 
-    def test_ru_ru_returns_ru(self):
-        assert deps._lang_from_accept_language("ru-RU,ru;q=0.9") == "ru"
+    def test_returns_ru_when_default_language_is_ru(self):
+        with patch.object(config, "DEFAULT_LANGUAGE", "ru"):
+            assert deps._lang_from_accept_language("en-US") == "ru"
+            assert deps._lang_from_accept_language(None) == "ru"
 
-    def test_en_us_returns_en(self):
-        assert deps._lang_from_accept_language("en-US") == "en"
-
-    def test_invalid_fallback_to_en(self):
-        assert deps._lang_from_accept_language("zz") == "en"
-        assert deps._lang_from_accept_language("x") == "en"
+    def test_returns_en_when_default_language_is_en(self):
+        with patch.object(config, "DEFAULT_LANGUAGE", "en"):
+            assert deps._lang_from_accept_language("ru-RU") == "en"
+            assert deps._lang_from_accept_language(None) == "en"
 
 
 class TestAuthErrorDetail:

tests/test_app.py

@@ -23,6 +23,41 @@ def test_health(client):
     assert r.json() == {"status": "ok"}
 
 
+def test_health_has_vary_accept_language(client):
+    """NoCacheStaticMiddleware adds Vary: Accept-Language to all responses."""
+    r = client.get("/health")
+    assert r.status_code == 200
+    assert "accept-language" in r.headers.get("vary", "").lower()
+
+
+def test_app_static_has_no_store_and_vary(client):
+    """Static files under /app get Cache-Control: no-store and Vary: Accept-Language."""
+    r = client.get("/app/")
+    if r.status_code != 200:
+        r = client.get("/app")
+    assert r.status_code == 200, "webapp static mount should serve index at /app or /app/"
+    assert r.headers.get("cache-control") == "no-store"
+    assert "accept-language" in r.headers.get("vary", "").lower()
+
+
+def test_app_js_has_no_store(client):
+    """JS and all static under /app get Cache-Control: no-store."""
+    r = client.get("/app/js/main.js")
+    assert r.status_code == 200
+    assert r.headers.get("cache-control") == "no-store"
+
+
+def test_app_config_js_returns_lang_from_default_language(client):
+    """GET /app/config.js returns JS setting window.__DT_LANG from config.DEFAULT_LANGUAGE."""
+    r = client.get("/app/config.js")
+    assert r.status_code == 200
+    assert r.headers.get("content-type", "").startswith("application/javascript")
+    assert r.headers.get("cache-control") == "no-store"
+    body = r.text
+    assert "window.__DT_LANG" in body
+    assert config.DEFAULT_LANGUAGE in body
+
+
 def test_duties_invalid_date_format(client):
     r = client.get("/api/duties", params={"from": "2025-01-01", "to": "invalid"})
     assert r.status_code == 400

tests/test_i18n.py

@@ -1,48 +1,46 @@
 """Unit tests for duty_teller.i18n: get_lang, t, fallback to en."""
 
-from unittest.mock import MagicMock
-
+from unittest.mock import MagicMock, patch
 
+import duty_teller.config as config
 from duty_teller.i18n import get_lang, t
 
 
-def test_get_lang_none_returns_en():
-    assert get_lang(None) == "en"
+def test_get_lang_always_returns_default_language():
+    """get_lang ignores user and always returns config.DEFAULT_LANGUAGE."""
+    assert get_lang(None) == config.DEFAULT_LANGUAGE
+    user_ru = MagicMock()
+    user_ru.language_code = "ru"
+    assert get_lang(user_ru) == config.DEFAULT_LANGUAGE
+    user_en = MagicMock()
+    user_en.language_code = "en"
+    assert get_lang(user_en) == config.DEFAULT_LANGUAGE
+    user_any = MagicMock(spec=[])
+    assert get_lang(user_any) == config.DEFAULT_LANGUAGE
 
 
-def test_get_lang_ru_returns_ru():
-    user = MagicMock()
-    user.language_code = "ru"
-    assert get_lang(user) == "ru"
+def test_get_lang_returns_ru_when_default_language_is_ru():
+    """When DEFAULT_LANGUAGE is ru, get_lang returns 'ru' regardless of user."""
+    with patch("duty_teller.i18n.core.config") as mock_cfg:
+        mock_cfg.DEFAULT_LANGUAGE = "ru"
+        from duty_teller.i18n.core import get_lang as core_get_lang
+
+        assert core_get_lang(None) == "ru"
+        user = MagicMock()
+        user.language_code = "en"
+        assert core_get_lang(user) == "ru"
 
 
-def test_get_lang_ru_ru_returns_ru():
-    user = MagicMock()
-    user.language_code = "ru-RU"
-    assert get_lang(user) == "ru"
+def test_get_lang_returns_en_when_default_language_is_en():
+    """When DEFAULT_LANGUAGE is en, get_lang returns 'en' regardless of user."""
+    with patch("duty_teller.i18n.core.config") as mock_cfg:
+        mock_cfg.DEFAULT_LANGUAGE = "en"
+        from duty_teller.i18n.core import get_lang as core_get_lang
 
-
-def test_get_lang_en_returns_en():
-    user = MagicMock()
-    user.language_code = "en"
-    assert get_lang(user) == "en"
-
-
-def test_get_lang_uk_returns_en():
-    user = MagicMock()
-    user.language_code = "uk"
-    assert get_lang(user) == "en"
-
-
-def test_get_lang_empty_returns_en():
-    user = MagicMock()
-    user.language_code = ""
-    assert get_lang(user) == "en"
-
-
-def test_get_lang_missing_attr_returns_en():
-    user = MagicMock(spec=[])  # no language_code
-    assert get_lang(user) == "en"
+        assert core_get_lang(None) == "en"
+        user = MagicMock()
+        user.language_code = "ru"
+        assert core_get_lang(user) == "en"
 
 
 def test_t_en_start_greeting():

tests/test_telegram_auth.py

@@ -1,5 +1,8 @@
 """Tests for duty_teller.api.telegram_auth.validate_init_data and validate_init_data_with_reason."""
 
+from unittest.mock import patch
+
+import duty_teller.config as config
 from duty_teller.api.telegram_auth import (
     validate_init_data,
     validate_init_data_with_reason,
@@ -52,7 +55,7 @@ def test_user_without_username_returns_none_from_validate_init_data():
 
 
 def test_user_without_username_but_with_id_succeeds_with_reason():
-    """With validate_init_data_with_reason, valid user.id is enough; username may be None."""
+    """With validate_init_data_with_reason, valid user.id is enough; lang is DEFAULT_LANGUAGE."""
     bot_token = "123:ABC"
     user = {"id": 456, "first_name": "Test", "language_code": "ru"}
     init_data = make_init_data(user, bot_token)
@@ -62,11 +65,11 @@ def test_user_without_username_but_with_id_succeeds_with_reason():
     assert telegram_user_id == 456
     assert username is None
     assert reason == "ok"
-    assert lang == "ru"
+    assert lang == config.DEFAULT_LANGUAGE
 
 
 def test_user_without_id_returns_no_user_id():
-    """When user object exists but has no 'id', return no_user_id."""
+    """When user object exists but has no 'id', return no_user_id; lang is DEFAULT_LANGUAGE."""
     bot_token = "123:ABC"
     user = {"first_name": "Test"}  # no id
     init_data = make_init_data(user, bot_token)
@@ -76,7 +79,17 @@ def test_user_without_id_returns_no_user_id():
     assert telegram_user_id is None
     assert username is None
     assert reason == "no_user_id"
-    assert lang == "en"
+    assert lang == config.DEFAULT_LANGUAGE
+
+
+def test_validate_init_data_with_reason_returns_default_language_ignoring_user_lang():
+    """Returned lang is always config.DEFAULT_LANGUAGE, not user.language_code."""
+    with patch("duty_teller.api.telegram_auth.config.DEFAULT_LANGUAGE", "ru"):
+        user = {"id": 1, "first_name": "U", "language_code": "en"}
+        init_data = make_init_data(user, "123:ABC")
+        _, _, reason, lang = validate_init_data_with_reason(init_data, "123:ABC")
+        assert reason == "ok"
+        assert lang == "ru"
 
 
 def test_empty_init_data_returns_none():