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>

docs/guides/letzshop-marketplace-api.md

@@ -1,17 +1,118 @@
-
-# Letzshop Development Documentation
+# Letzshop Marketplace Integration
 
 ## Introduction
 
-Welcome to the Letzshop Development page. Here you’ll find details on:
+This guide covers the Wizamart platform's integration with the Letzshop marketplace, including:
 
-- GraphQL API
-- Authentication
-- Playground
-- Deprecations
-- Order management
-- Event system
-- Data structures [1](https://letzshop.lu/en/dev)
+- **Product Export**: Generate Letzshop-compatible CSV files from your product catalog
+- **Order Import**: Fetch and manage orders from Letzshop
+- **Fulfillment Sync**: Confirm/reject orders, set tracking numbers
+- **GraphQL API Reference**: Direct API access for advanced use cases
+
+---
+
+## Product Export
+
+### Overview
+
+The Wizamart platform can export your products to Letzshop-compatible CSV format (Google Shopping feed format). This allows you to:
+
+- Upload your product catalog to Letzshop marketplace
+- Generate feeds in multiple languages (English, French, German)
+- Include all required Letzshop fields automatically
+
+### API Endpoints
+
+#### Vendor Export
+
+```http
+GET /api/v1/vendor/letzshop/export?language=fr
+Authorization: Bearer <vendor_token>
+```
+
+**Parameters:**
+
+| Parameter | Type | Default | Description |
+|-----------|------|---------|-------------|
+| `language` | string | `en` | Language for title/description (`en`, `fr`, `de`) |
+| `include_inactive` | bool | `false` | Include inactive products |
+
+**Response:** CSV file download (`vendor_code_letzshop_export.csv`)
+
+#### Admin Export
+
+```http
+GET /api/v1/admin/letzshop/export?vendor_id=1&language=fr
+Authorization: Bearer <admin_token>
+```
+
+**Additional Parameters:**
+
+| Parameter | Type | Description |
+|-----------|------|-------------|
+| `vendor_id` | int | Required. Vendor ID to export |
+
+### CSV Format
+
+The export generates a tab-separated CSV file with these columns:
+
+| Column | Description | Example |
+|--------|-------------|---------|
+| `id` | Product SKU | `PROD-001` |
+| `title` | Product title (localized) | `Wireless Headphones` |
+| `description` | Product description (localized) | `High-quality...` |
+| `link` | Product URL | `https://shop.example.com/product/123` |
+| `image_link` | Main product image | `https://cdn.example.com/img.jpg` |
+| `additional_image_link` | Additional images (comma-separated) | `img2.jpg,img3.jpg` |
+| `availability` | Stock status | `in stock` / `out of stock` |
+| `price` | Regular price with currency | `49.99 EUR` |
+| `sale_price` | Sale price with currency | `39.99 EUR` |
+| `brand` | Brand name | `TechBrand` |
+| `gtin` | Global Trade Item Number | `0012345678901` |
+| `mpn` | Manufacturer Part Number | `TB-WH-001` |
+| `google_product_category` | Google category ID | `Electronics > Audio` |
+| `condition` | Product condition | `new` / `used` / `refurbished` |
+| `atalanda:tax_rate` | Luxembourg VAT rate | `17` |
+
+### Multi-Language Support
+
+Products are exported with localized content based on the `language` parameter:
+
+```bash
+# French export
+curl -H "Authorization: Bearer $TOKEN" \
+  "https://api.example.com/api/v1/vendor/letzshop/export?language=fr"
+
+# German export
+curl -H "Authorization: Bearer $TOKEN" \
+  "https://api.example.com/api/v1/vendor/letzshop/export?language=de"
+
+# English export (default)
+curl -H "Authorization: Bearer $TOKEN" \
+  "https://api.example.com/api/v1/vendor/letzshop/export?language=en"
+```
+
+If a translation is not available for the requested language, the system falls back to English, then to any available translation.
+
+### Using the Export
+
+1. **Navigate to Letzshop** in your vendor dashboard
+2. **Click the Export tab**
+3. **Select your language** (French, German, or English)
+4. **Click "Download CSV"**
+5. **Upload to Letzshop** via their merchant portal
+
+---
+
+## Order Integration
+
+For details on order import and fulfillment, see [Letzshop Order Integration](./letzshop-order-integration.md).
+
+---
+
+## Letzshop GraphQL API Reference
+
+The following sections document the Letzshop GraphQL API for direct integration.
 
 ---