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.

duty_teller/run.py

@@ -28,7 +28,7 @@ def _set_default_menu_button_webapp() -> None:
     payload = {
         "menu_button": {
             "type": "web_app",
-            "text": "Календарь",
+            "text": "Calendar",
             "web_app": {"url": menu_url},
         }
     }