Add real country flags to language selector

- Replace emoji flags with SVG-based country flags
- German flag: Black-Red-Gold (official colors)
- USA flag: Red-White-Blue with stars
- Dynamic flag switching on language change
- Applied to both dashboard and leaderboard
- Database achievements now support English translations
- Extended achievements table with name_en and description_en columns
- Updated API routes to return English translations
- Simplified frontend translation system to use database translations

public/index.html

@@ -25,8 +25,8 @@
         <!-- Language Selector -->
         <div class="language-selector">
             <select id="languageSelect" onchange="changeLanguage()">
-                <option value="de">🇩🇪 Deutsch</option>
-                <option value="en">🇺🇸 English</option>
+                <option value="de" data-flag="🇩🇪">Deutsch</option>
+                <option value="en" data-flag="🇺🇸">English</option>
             </select>
         </div>