Add support for user's native language in VocabCourseListView

- Introduced a new option for selecting the user's native language in the course selection dropdown.
- Implemented logic to load the user's native language based on the UI language and map it to the corresponding language ID.
- Updated internationalization files to include strings for the user's native language in both German and English, enhancing the user experience.

frontend/src/i18n/locales/de/socialnetwork.json

@@ -355,6 +355,7 @@
                 "targetLanguage": "Zielsprache",
                 "nativeLanguage": "Muttersprache",
                 "allNativeLanguages": "Alle Muttersprachen",
+                "myNativeLanguage": "Meine Muttersprache",
                 "forAllLanguages": "Für alle Sprachen",
                 "optional": "Optional",
                 "invalidCode": "Ungültiger Code",

frontend/src/i18n/locales/en/socialnetwork.json

@@ -355,6 +355,7 @@
                 "targetLanguage": "Target Language",
                 "nativeLanguage": "Native Language",
                 "allNativeLanguages": "All Native Languages",
+                "myNativeLanguage": "My Native Language",
                 "forAllLanguages": "For All Languages",
                 "optional": "Optional",
                 "invalidCode": "Invalid code",