feat: add multi-language (i18n) support for vendor dashboard and storefront

- Add database fields for language preferences:
  - Vendor: dashboard_language, storefront_language, storefront_languages
  - User: preferred_language
  - Customer: preferred_language

- Add language middleware for request-level language detection:
  - Cookie-based persistence
  - Browser Accept-Language fallback
  - Vendor storefront language constraints

- Add language API endpoints (/api/v1/language/*):
  - POST /set - Set language preference
  - GET /current - Get current language info
  - GET /list - List available languages
  - DELETE /clear - Clear preference

- Add i18n utilities (app/utils/i18n.py):
  - JSON-based translation loading
  - Jinja2 template integration
  - Language resolution helpers

- Add reusable language selector macros for templates
- Add languageSelector() Alpine.js component
- Add translation files (en, fr, de, lb) in static/locales/
- Add architecture rules documentation for language implementation
- Update marketplace-product-detail.js to use native language names

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

static/vendor/js/init-alpine.js

@@ -118,4 +118,50 @@ function data() {
             }
         }
     };
-}
+}
+
+/**
+ * Language Selector Component
+ * Alpine.js component for language switching in vendor dashboard
+ */
+function languageSelector(currentLang, enabledLanguages) {
+    return {
+        isLangOpen: false,
+        currentLang: currentLang || 'fr',
+        languages: enabledLanguages || ['en', 'fr', 'de', 'lb'],
+        languageNames: {
+            'en': 'English',
+            'fr': 'Français',
+            'de': 'Deutsch',
+            'lb': 'Lëtzebuergesch'
+        },
+        languageFlags: {
+            'en': 'gb',
+            'fr': 'fr',
+            'de': 'de',
+            'lb': 'lu'
+        },
+        async setLanguage(lang) {
+            if (lang === this.currentLang) {
+                this.isLangOpen = false;
+                return;
+            }
+            try {
+                const response = await fetch('/api/v1/language/set', {
+                    method: 'POST',
+                    headers: { 'Content-Type': 'application/json' },
+                    body: JSON.stringify({ language: lang })
+                });
+                if (response.ok) {
+                    this.currentLang = lang;
+                    window.location.reload();
+                }
+            } catch (error) {
+                console.error('Failed to set language:', error);
+            }
+            this.isLangOpen = false;
+        }
+    };
+}
+
+window.languageSelector = languageSelector;