Add native language support in vocab course management

- Introduced a new field for native language in the VocabCourse model to allow learners to specify their native language.
- Updated the VocabService to handle native language during course creation and retrieval, including filtering options.
- Enhanced the database schema to include foreign key constraints for native language.
- Updated frontend components to support native language selection and display in course listings.
- Added internationalization strings for native language features in both German and English.

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

@@ -352,6 +352,11 @@
                 "shareCode": "Share-Code",
                 "searchPlaceholder": "Kurs suchen...",
                 "allLanguages": "Alle Sprachen",
+                "targetLanguage": "Zielsprache",
+                "nativeLanguage": "Muttersprache",
+                "allNativeLanguages": "Alle Muttersprachen",
+                "forAllLanguages": "Für alle Sprachen",
+                "optional": "Optional",
                 "invalidCode": "Ungültiger Code",
                 "courseNotFound": "Kurs nicht gefunden"
             }

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

@@ -352,6 +352,11 @@
                 "shareCode": "Share Code",
                 "searchPlaceholder": "Search courses...",
                 "allLanguages": "All Languages",
+                "targetLanguage": "Target Language",
+                "nativeLanguage": "Native Language",
+                "allNativeLanguages": "All Native Languages",
+                "forAllLanguages": "For All Languages",
+                "optional": "Optional",
                 "invalidCode": "Invalid code",
                 "courseNotFound": "Course not found"
             }