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_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"
+    )