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/handlers/errors.py

@@ -5,6 +5,9 @@ import logging
 from telegram import Update
 from telegram.ext import ContextTypes
 
+import duty_teller.config as config
+from duty_teller.i18n import get_lang, t
+
 logger = logging.getLogger(__name__)
 
 
@@ -13,4 +16,6 @@ async def error_handler(
 ) -> None:
     logger.exception("Exception while handling an update")
     if isinstance(update, Update) and update.effective_message:
-        await update.effective_message.reply_text("Произошла ошибка. Попробуйте позже.")
+        user = getattr(update, "effective_user", None)
+        lang = get_lang(user) if user else config.DEFAULT_LANGUAGE
+        await update.effective_message.reply_text(t(lang, "errors.generic"))