Add community demo: error localization engine (Closes #1761)

community/error-localization-engine/.env.example

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+LINGO_API_KEY=your_api_key_here
+PORT=3000

community/error-localization-engine/.gitignore

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+node_modules/
+.env
+*.log
+.DS_Store

community/error-localization-engine/README.md

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+# Error Message & Validation Localization Engine
+
+A production-grade demo showing how to automatically localize backend error and validation messages using [Lingo.dev](https://lingo.dev).
+
+## 🚀 The Core Problem
+
+Most applications translate UI labels but leave backend error messages in English. This creates a disjointed experience for international users (e.g., getting a "Password too short" error in English while browsing a Spanish site).
+
+This usage fixes that by intercepting errors and dynamically localizing them before they reach the client.
+
+## 🛠️ Tech Stack
+
+- **Node.js**: Runtime environment.
+- **Express**: Web server and routing.
+- **Lingo.dev SDK**: AI-powered localization engine.
+
+## 🏃‍♂️ How to Run
+
+1. **Install Dependencies**
+   ```bash
+   npm install
+   ```
+
+2. **Configure Environment**
+   Create a `.env` file and add your Lingo.dev API key:
+   ```bash
+   LINGO_API_KEY=your_api_key_here
+   ```
+
+3. **Start the Server**
+   ```bash
+   npm start
+   ```
+
+4. **Open the Demo**
+   - Open your browser and go to: `http://localhost:3000`
+   - Select a language from the dropdown
+   - Try the test cases to see localized errors!
+
+## 🧪 Testing the API
+
+The API detects language preference via:
+1. `?lang=xx` query parameter (High priority)
+2. `Accept-Language` header (Low priority)
+
+### 1. English (Default)
+**Request:**
+```bash
+curl -X POST http://localhost:3000/api/auth/signup \
+  -H "Content-Type: application/json" \
+  -d '{"email": "invalid-email", "password": "123"}'
+```
+**Response:**
+```json
+{
+  "error": "Validation Failed",
+  "message": "Please provide a valid email address."
+}
+```
+
+### 2. Spanish (via Query Param)
+**Request:**
+```bash
+curl -X POST "http://localhost:3000/api/auth/signup?lang=es" \
+  -H "Content-Type: application/json" \
+  -d '{"email": "invalid-email", "password": "123"}'
+```
+**Response (Localized):**
+```json
+{
+  "error": "Falló la validación",
+  "message": "Por favor proporcione una dirección de correo electrónico válida."
+}
+```
+
+### 3. French (via Header)
+**Request:**
+```bash
+curl -X POST http://localhost:3000/api/auth/signup \
+  -H "Content-Type: application/json" \
+  -H "Accept-Language: fr" \
+  -d '{"email": "test@example.com", "password": "short"}'
+```
+**Response (Localized):**
+```json
+{
+  "error": "Échec de la validation",
+  "message": "Le mot de passe doit contenir au moins 6 caractères."
+}
+```

community/error-localization-engine/lingoClient.js

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+const { LingoDotDevEngine } = require("lingo.dev/sdk");
+
+// Initialize the Lingo.dev engine if KEY is present
+const apiKey = process.env.LINGO_API_KEY;
+let lingoDotDev = null;
+
+if (apiKey && apiKey !== 'your_api_key_here') {
+    lingoDotDev = new LingoDotDevEngine({ apiKey });
+}
+
+/**
+ * Localizes an error message or object to the target language.
+ * @param {string|object} content - The error message or object to localize.
+ * @param {string} targetLocale - The target language code (e.g., 'es', 'fr').
+ * @returns {Promise<string|object>} - The localized content.
+ */
+async function localizeError(content, targetLocale) {
+  if (!targetLocale || targetLocale.startsWith('en')) {
+    return content;
+  }
+
+  // MOCK MODE: If no API key is set, return mock translations
+  if (!lingoDotDev) {
+    console.log(`[Mock] Localizing to ${targetLocale}:`, JSON.stringify(content));
+    const isString = typeof content === 'string';
+
+    // Simple mock translation function
+    const mockTranslate = (str) => `[${targetLocale}] ${str}`;
+
+    if (isString) {
+        return mockTranslate(content);
+    } else {
+        const translated = {};
+        for (const [key, value] of Object.entries(content)) {
+            if (typeof value === 'string') {
+                translated[key] = mockTranslate(value);
+            } else {
+                translated[key] = value;
+            }
+        }
+        return translated;
+    }
+  }
+
+  try {
+    const isString = typeof content === 'string';
+    const input = isString ? { message: content } : content;
+
+    const translated = await lingoDotDev.localizeObject(input, {
+        sourceLocale: "en",
+        targetLocale: targetLocale,
+    });
+
+    return isString ? translated.message : translated;
+
+  } catch (error) {
+    console.error("Localization failed:", error.message);
+    return content;
+  }
+}
+
+module.exports = { localizeError };

community/error-localization-engine/middleware/localizer.js

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+/**
+ * Middleware to extract the target language from the request.
+ * Prioritizes 'lang' query parameter over 'Accept-Language' header.
+ * Defaults to 'en' (English).
+ */
+function localizer(req, res, next) {
+    // 1. Check query param ?lang=xx
+    let lang = req.query.lang;
+
+    // 2. Check Accept-Language header
+    if (!lang && req.headers['accept-language']) {
+        // Simple parse: take the first language code (e.g., 'en-US,en;q=0.9' -> 'en-US')
+        // And maybe just take the first 2 chars or the full code. 
+        // Lingo.dev likely supports standard BCP-47.
+        const acceptLang = req.headers['accept-language'].split(',')[0].trim();
+        lang = acceptLang;
+    }
+
+    // 3. Default to English
+    req.targetLang = lang || 'en';
+
+    // Log for debugging
+    console.log(`[Request] ${req.method} ${req.path} - Target Lang: ${req.targetLang}`);
+
+    next();
+}
+
+module.exports = localizer;