feat(i18n): Implement server-side internationalization support

- Added ServerI18nManager to handle translations and locale settings on the server.
- Integrated server-side translations into templates, reducing language flashing on initial load.
- Created API endpoints for setting and getting user language preferences.
- Enhanced client-side i18n handling to work seamlessly with server-rendered content.
- Updated various templates to utilize the new translation system.
- Added mixed i18n handler to coordinate server and client translations, improving user experience.
- Expanded translation files to include initialization messages for various components.

py/routes/misc_routes.py

@@ -112,6 +112,10 @@ class MiscRoutes:
         
         # Add new route for checking if a model exists in the library
         app.router.add_get('/api/check-model-exists', MiscRoutes.check_model_exists)
+        
+        # Language settings endpoints
+        app.router.add_post('/api/set-language', MiscRoutes.set_language)
+        app.router.add_get('/api/get-language', MiscRoutes.get_language)
 
     @staticmethod
     async def clear_cache(request):
@@ -697,3 +701,69 @@ class MiscRoutes:
                 'success': False,
                 'error': str(e)
             }, status=500)
+
+    @staticmethod
+    async def set_language(request):
+        """
+        Set user language preference
+        
+        Expects a JSON body with:
+        {
+            "language": "en" | "zh-CN" | "zh-TW" | "ru" | "de" | "ja" | "ko" | "fr" | "es"
+        }
+        """
+        try:
+            data = await request.json()
+            language = data.get('language')
+            
+            if not language:
+                return web.json_response({
+                    'success': False,
+                    'error': 'Missing language parameter'
+                }, status=400)
+            
+            # Validate language code
+            supported_languages = ['en', 'zh-CN', 'zh-TW', 'ru', 'de', 'ja', 'ko', 'fr', 'es']
+            if language not in supported_languages:
+                return web.json_response({
+                    'success': False,
+                    'error': f'Unsupported language: {language}. Supported languages: {", ".join(supported_languages)}'
+                }, status=400)
+            
+            # Save language setting
+            settings.set('language', language)
+            
+            return web.json_response({
+                'success': True,
+                'message': f'Language set to {language}',
+                'language': language
+            })
+            
+        except Exception as e:
+            logger.error(f"Failed to set language: {e}", exc_info=True)
+            return web.json_response({
+                'success': False,
+                'error': str(e)
+            }, status=500)
+    
+    @staticmethod
+    async def get_language(request):
+        """
+        Get current user language preference
+        
+        Returns the current language setting from settings
+        """
+        try:
+            current_language = settings.get('language', 'en')
+            
+            return web.json_response({
+                'success': True,
+                'language': current_language
+            })
+            
+        except Exception as e:
+            logger.error(f"Failed to get language: {e}", exc_info=True)
+            return web.json_response({
+                'success': False,
+                'error': str(e)
+            }, status=500)