Add Vocab Trainer feature with routing, database schema, and translations

- Introduced Vocab Trainer functionality, including new routes for managing languages and chapters.
- Implemented database schema for vocab-related tables to ensure data integrity.
- Updated navigation and UI components to include Vocab Trainer in the social network menu.
- Added translations for Vocab Trainer in both German and English locales, enhancing user accessibility.

frontend/src/components/AppNavigation.vue

@@ -27,7 +27,7 @@
               :style="`background-image:url('/images/icons/${subitem.icon}')`"
               class="submenu-icon"
             >&nbsp;</span>
-            <span>{{ $t(`navigation.m-${key}.${subkey}`) }}</span>
+            <span>{{ subitem?.label || $t(`navigation.m-${key}.${subkey}`) }}</span>
             <span
               v-if="subkey === 'forum' || subitem.children"
               class="subsubmenu"
@@ -62,7 +62,7 @@
                   :style="`background-image:url('/images/icons/${subsubitem.icon}')`"
                   class="submenu-icon"
                 >&nbsp;</span>
-                <span>{{ $t(`navigation.m-${key}.m-${subkey}.${subsubkey}`) }}</span>
+                <span>{{ subsubitem?.label || $t(`navigation.m-${key}.m-${subkey}.${subsubkey}`) }}</span>
               </li>
             </ul>
           </li>