Refactor navigation structure to enhance language learning features

- Renamed 'vocabtrainer' to 'sprachenlernen' in the navigation structure for better clarity.
- Introduced a nested structure under 'sprachenlernen' for 'vocabtrainer' and 'sprachkurse', improving organization of language-related resources.
- Updated internationalization files for both German and English to reflect the new naming and structure, ensuring consistency across the application.

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

@@ -20,7 +20,7 @@
             "usersearch": "Benutzersuche",
             "forum": "Forum",
             "gallery": "Galerie",
-            "vocabtrainer": "Vokabeltrainer",
+            "sprachenlernen": "Sprachen lernen",
             "blockedUsers": "Blockierte Benutzer",
             "oneTimeInvitation": "Einmal-Einladungen",
             "diary": "Tagebuch",
@@ -29,8 +29,12 @@
                 "pictures": "Bilder",
                 "videos": "Videos"
             },
-            "m-vocabtrainer": {
-                "newLanguage": "Neue Sprache"
+            "m-sprachenlernen": {
+                "vocabtrainer": "Vokabeltrainer",
+                "sprachkurse": "Sprachkurse",
+                "m-vocabtrainer": {
+                    "newLanguage": "Neue Sprache"
+                }
             }
         },
         "m-minigames": {

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

@@ -20,7 +20,7 @@
             "usersearch": "User search",
             "forum": "Forum",
             "gallery": "Gallery",
-            "vocabtrainer": "Vocabulary trainer",
+            "sprachenlernen": "Learn Languages",
             "blockedUsers": "Blocked users",
             "oneTimeInvitation": "One-time invitations",
             "diary": "Diary",
@@ -29,8 +29,12 @@
                 "pictures": "Pictures",
                 "videos": "Videos"
             },
-            "m-vocabtrainer": {
-                "newLanguage": "New language"
+            "m-sprachenlernen": {
+                "vocabtrainer": "Vocabulary trainer",
+                "sprachkurse": "Language courses",
+                "m-vocabtrainer": {
+                    "newLanguage": "New language"
+                }
             }
         },
         "m-minigames": {