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 = [