Refactor native language filtering in VocabService and update frontend handling

- Simplified the logic for filtering courses by native language in VocabService, allowing for better handling of undefined and null values.
- Enhanced the VocabCourseListView to clarify the behavior of nativeLanguageId based on user selection, ensuring accurate course retrieval based on language preferences.
- Improved comments in both files for better understanding of the filtering logic and its implications on course visibility.

frontend/src/views/social/VocabCourseListView.vue

@@ -172,9 +172,21 @@ export default {
         if (this.selectedLanguageId) {
           params.languageId = this.selectedLanguageId;
         }
+        // Nur nativeLanguageId senden, wenn explizit eine Sprache ausgewählt wurde
+        // Leer bedeutet: zeige alle Kurse
+        // "null" bedeutet: zeige nur Kurse ohne Muttersprache
+        // Eine ID bedeutet: zeige nur Kurse für diese Muttersprache
         if (this.selectedNativeLanguageId !== '') {
-          params.nativeLanguageId = this.selectedNativeLanguageId === 'null' ? null : this.selectedNativeLanguageId;
+          if (this.selectedNativeLanguageId === 'null') {
+            // Explizit Kurse ohne Muttersprache anfordern
+            params.nativeLanguageId = null;
+          } else {
+            // Spezifische Muttersprache
+            params.nativeLanguageId = this.selectedNativeLanguageId;
+          }
         }
+        // Wenn selectedNativeLanguageId leer ist, wird nativeLanguageId nicht gesetzt
+        // und das Backend zeigt alle Kurse an
         if (this.searchTerm.trim()) {
           params.search = this.searchTerm.trim();
         }