Add bilingual support (German/English) with language selector

- Added language selector in top-left corner
- Implemented data attributes for all translatable text
- Created language management system with localStorage persistence
- Updated all JavaScript functions to support both languages
- Added translations for notifications, error messages, and UI elements
- Maintained existing functionality while adding language switching

public/css/leaderboard.css

@@ -24,6 +24,38 @@ body {
     min-height: 100vh;
 }
 
+/* Language Selector */
+.language-selector {
+    position: fixed;
+    top: 2rem;
+    left: 2rem;
+    z-index: 1000;
+}
+
+.language-selector select {
+    padding: 0.5rem 1rem;
+    background: rgba(15, 23, 42, 0.9);
+    border: 1px solid #334155;
+    border-radius: 0.5rem;
+    color: #ffffff;
+    font-size: 0.9rem;
+    font-weight: 500;
+    cursor: pointer;
+    transition: all 0.2s ease;
+    backdrop-filter: blur(10px);
+}
+
+.language-selector select:hover {
+    border-color: #00d4ff;
+    box-shadow: 0 0 0 2px rgba(0, 212, 255, 0.1);
+}
+
+.language-selector select:focus {
+    outline: none;
+    border-color: #00d4ff;
+    box-shadow: 0 0 0 3px rgba(0, 212, 255, 0.2);
+}
+
 .header-section {
     text-align: center;
     margin-bottom: 3rem;