Add internationalization support and enhance language handling

- Introduced a new i18n module for managing translations and language normalization, supporting both Russian and English.
- Updated various handlers and services to utilize the new translation functions for user-facing messages, improving user experience based on language preferences.
- Enhanced error handling and response messages to be language-aware, ensuring appropriate feedback is provided to users in their preferred language.
- Added tests for the i18n module to validate language detection and translation functionality.
- Updated the example environment file to include a default language configuration.

tests/test_app.py

@@ -19,13 +19,15 @@ def client():
 def test_duties_invalid_date_format(client):
     r = client.get("/api/duties", params={"from": "2025-01-01", "to": "invalid"})
     assert r.status_code == 400
-    assert "YYYY-MM-DD" in r.json()["detail"]
+    detail = r.json()["detail"]
+    assert "from" in detail.lower() and "to" in detail.lower()
 
 
 def test_duties_from_after_to(client):
     r = client.get("/api/duties", params={"from": "2025-02-01", "to": "2025-01-01"})
     assert r.status_code == 400
-    assert "from" in r.json()["detail"].lower() or "позже" in r.json()["detail"]
+    detail = r.json()["detail"].lower()
+    assert "from" in detail or "to" in detail or "after" in detail or "позже" in detail
 
 
 @patch("duty_teller.api.dependencies._is_private_client")
@@ -54,7 +56,7 @@ def test_duties_200_when_skip_auth(mock_fetch, client):
 
 @patch("duty_teller.api.dependencies.validate_init_data_with_reason")
 def test_duties_403_when_init_data_invalid(mock_validate, client):
-    mock_validate.return_value = (None, "hash_mismatch")
+    mock_validate.return_value = (None, "hash_mismatch", "en")
     r = client.get(
         "/api/duties",
         params={"from": "2025-01-01", "to": "2025-01-31"},
@@ -62,13 +64,18 @@ def test_duties_403_when_init_data_invalid(mock_validate, client):
     )
     assert r.status_code == 403
     detail = r.json()["detail"]
-    assert "авторизации" in detail or "Неверные" in detail or "Неверная" in detail
+    assert (
+        "signature" in detail.lower()
+        or "авторизации" in detail
+        or "Неверные" in detail
+        or "Неверная" in detail
+    )
 
 
 @patch("duty_teller.api.dependencies.validate_init_data_with_reason")
 @patch("duty_teller.api.dependencies.config.can_access_miniapp")
 def test_duties_403_when_username_not_allowed(mock_can_access, mock_validate, client):
-    mock_validate.return_value = ("someuser", "ok")
+    mock_validate.return_value = ("someuser", "ok", "en")
     mock_can_access.return_value = False
     with patch("duty_teller.api.app.fetch_duties_response") as mock_fetch:
         r = client.get(
@@ -77,14 +84,16 @@ def test_duties_403_when_username_not_allowed(mock_can_access, mock_validate, cl
             headers={"X-Telegram-Init-Data": "user=xxx&hash=yyy"},
         )
     assert r.status_code == 403
-    assert "Доступ запрещён" in r.json()["detail"]
+    assert (
+        "Access denied" in r.json()["detail"] or "Доступ запрещён" in r.json()["detail"]
+    )
     mock_fetch.assert_not_called()
 
 
 @patch("duty_teller.api.dependencies.validate_init_data_with_reason")
 @patch("duty_teller.api.dependencies.config.can_access_miniapp")
 def test_duties_200_with_allowed_user(mock_can_access, mock_validate, client):
-    mock_validate.return_value = ("alloweduser", "ok")
+    mock_validate.return_value = ("alloweduser", "ok", "en")
     mock_can_access.return_value = True
     with patch("duty_teller.api.app.fetch_duties_response") as mock_fetch:
         mock_fetch.return_value = [

tests/test_i18n.py

@@ -0,0 +1,76 @@
+"""Unit tests for duty_teller.i18n: get_lang, t, fallback to en."""
+
+from unittest.mock import MagicMock
+
+
+from duty_teller.i18n import get_lang, t
+
+
+def test_get_lang_none_returns_en():
+    assert get_lang(None) == "en"
+
+
+def test_get_lang_ru_returns_ru():
+    user = MagicMock()
+    user.language_code = "ru"
+    assert 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_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"
+
+
+def test_t_en_start_greeting():
+    result = t("en", "start.greeting")
+    assert "help" in result.lower()
+    assert "bot" in result.lower()
+
+
+def test_t_ru_start_greeting():
+    result = t("ru", "start.greeting")
+    assert "Привет" in result or "бот" in result or "календар" in result
+
+
+def test_t_fallback_to_en_when_key_missing_for_ru():
+    """When key exists in en but not in ru, fallback to en."""
+    result = t("ru", "start.greeting")
+    assert len(result) > 0
+    # start.greeting exists in both; use a key that might be en-only for fallback
+    result_any = t("ru", "errors.generic")
+    assert "error" in result_any.lower() or "ошибк" in result_any.lower()
+
+
+def test_t_placeholder_substitution():
+    assert t("en", "set_phone.saved", phone="+7 999") == "Phone saved: +7 999"
+    assert "999" in t("ru", "set_phone.saved", phone="+7 999")
+
+
+def test_t_unknown_lang_normalized_to_en():
+    assert "help" in t("de", "start.greeting").lower() or "Hi" in t(
+        "de", "start.greeting"
+    )