diff --git a/src/db/migrations/20260803000000_optimize_database_queries.js b/src/db/migrations/20260803000000_optimize_database_queries.js
new file mode 100644
index 0000000..8996c73
--- /dev/null
+++ b/src/db/migrations/20260803000000_optimize_database_queries.js
@@ -0,0 +1,34 @@
+'use strict';
+
+/**
+ * Add composite indexes and query optimizations (#570).
+ *
+ * Adds composite indexes for common query patterns combining filtering
+ * and sorting operations to improve query performance.
+ */
+
+exports.up = async function up(knex) {
+  // Recipients: list by airdrop with ordering
+  await knex.raw('CREATE INDEX IF NOT EXISTS idx_recipients_airdrop_created ON recipients (airdrop_id, created_at DESC)');
+
+  // Alerts: list by asset with status filtering and ordering
+  await knex.raw('CREATE INDEX IF NOT EXISTS idx_alerts_asset_status_created ON alerts (asset, status, created_at DESC)');
+
+  // Webhooks: active webhooks by event type (composite for filtering)
+  await knex.raw('CREATE INDEX IF NOT EXISTS idx_webhooks_active_event ON webhooks (active, event_type)');
+
+  // API key audit: filter by key and created_at range efficiently
+  await knex.raw('CREATE INDEX IF NOT EXISTS idx_api_key_audit_key_created ON api_key_audit_logs (key_id, created_at DESC)');
+};
+
+exports.down = async function down(knex) {
+  const indexes = [
+    'idx_recipients_airdrop_created',
+    'idx_alerts_asset_status_created',
+    'idx_webhooks_active_event',
+    'idx_api_key_audit_key_created',
+  ];
+  for (const idx of indexes) {
+    await knex.raw(`DROP INDEX IF EXISTS ${idx}`);
+  }
+};
diff --git a/src/middleware/errorHandler.js b/src/middleware/errorHandler.js
index c47476a..29ed2ac 100644
--- a/src/middleware/errorHandler.js
+++ b/src/middleware/errorHandler.js
@@ -3,6 +3,7 @@
 const AppError = require('../errors/AppError');
 const logger = require('../logger');
 const errorTracker = require('../services/errorTracker');
+const { translate, getLocale } = require('../services/i18n');
 
 /**
  * Standardized error response format (#491).
@@ -45,9 +46,10 @@ function notFoundHandler(req, _res, next) {
 function errorHandler(err, req, res, _next) {
   const isAppError = err instanceof AppError;
   const isPayloadTooLarge = !isAppError && (err.type === 'entity.too.large' || err.status === 413 || err.statusCode === 413);
+  const locale = getLocale(req);
   let status = 500;
   let code = 'INTERNAL_ERROR';
-  let message = 'An unexpected error occurred';
+  let message = translate('INTERNAL_ERROR', locale);
 
   if (isAppError) {
     status = err.statusCode;
@@ -56,14 +58,14 @@ function errorHandler(err, req, res, _next) {
   } else if (err.isCircuitBreakerError || err.name === 'CircuitBreakerOpenError') {
     status = 503;
     code = 'SERVICE_UNAVAILABLE';
-    message = err.message || 'Service temporarily unavailable';
+    message = err.message || translate('SERVICE_UNAVAILABLE', locale);
     if (err.retryAfter) {
       res.setHeader('Retry-After', String(err.retryAfter));
     }
   } else if (isPayloadTooLarge) {
     status = 413;
     code = 'PAYLOAD_TOO_LARGE';
-    message = 'Request body is too large';
+    message = translate('PAYLOAD_TOO_LARGE', locale);
   } else if (err.status || err.statusCode) {
     status = err.status || err.statusCode;
     const STATUS_CODES = { 400: 'VALIDATION_ERROR', 401: 'UNAUTHORIZED', 403: 'FORBIDDEN', 404: 'NOT_FOUND', 413: 'PAYLOAD_TOO_LARGE', 429: 'RATE_LIMITED', 503: 'SERVICE_UNAVAILABLE' };
@@ -91,8 +93,10 @@ function errorHandler(err, req, res, _next) {
 
   // #449 — Sanitize the message before sending to the client to prevent
   // stack traces, file paths, or other internal details from leaking.
+  // #571 — Localize standard error messages based on the client's language preference.
   const safeMessage = sanitizeMessage(message);
-  const error = { code, message: safeMessage, request_id: req.id };
+  const localizedMessage = translate(code, locale) !== code ? translate(code, locale) : safeMessage;
+  const error = { code, message: localizedMessage, request_id: req.id };
   if (isAppError && err.details && Object.keys(err.details).length > 0) {
     // Also sanitize details to prevent stack traces in nested fields.
     const safeDetails = {};
diff --git a/src/routes/airdrops.js b/src/routes/airdrops.js
index 71294d5..b372984 100644
--- a/src/routes/airdrops.js
+++ b/src/routes/airdrops.js
@@ -9,6 +9,7 @@ const { idempotencyMiddleware } = require("../services/idempotency");
 const logger = require("../logger");
 const AppError = require("../errors/AppError");
 const { flattenZodIssues, validate } = require("../middleware/validate");
+const { requireApiKey } = require("../middleware/auth");
 const {
   airdropCreateBodySchema,
   airdropRecipientsBodySchema,
@@ -300,6 +301,7 @@ async function parseCSV(buffer) {
 
 router.post(
   "/airdrops",
+  requireApiKey({ scopes: ["airdrops"] }),
   routeTimeout(),
   createAirdropLimit,
   idempotencyMiddleware("airdrop"),
@@ -343,6 +345,7 @@ router.get("/airdrops/:id", validateRouteIdParams, async (req, res, next) => {
 
 router.patch(
   "/airdrops/:id",
+  requireApiKey({ scopes: ["airdrops"] }),
   validateRouteIdParams,
   validateWithCurrentLedger(airdropUpdateBodySchema),
   async (req, res, next) => {
@@ -366,6 +369,7 @@ router.patch(
 
 router.delete(
   "/airdrops/:id",
+  requireApiKey({ scopes: ["airdrops"] }),
   validateRouteIdParams,
   async (req, res, next) => {
     try {
@@ -385,6 +389,7 @@ router.delete(
 
 router.post(
   "/airdrops/:id/cancel",
+  requireApiKey({ scopes: ["airdrops"] }),
   validateRouteIdParams,
   async (req, res, next) => {
     try {
@@ -404,6 +409,7 @@ router.post(
 
 router.post(
   "/airdrops/:id/recipients",
+  requireApiKey({ scopes: ["airdrops"] }),
   routeTimeout(),
   validateRouteIdParams,
   addRecipientsLimit,
diff --git a/src/services/i18n.js b/src/services/i18n.js
new file mode 100644
index 0000000..5b533c4
--- /dev/null
+++ b/src/services/i18n.js
@@ -0,0 +1,78 @@
+'use strict';
+
+/**
+ * Internationalization (i18n) service for error messages (#571).
+ *
+ * Supports error message localization based on Accept-Language header.
+ * Currently supports: English (en), Spanish (es), French (fr).
+ */
+
+const translations = {
+  en: {
+    UNAUTHORIZED: 'Missing or invalid API key',
+    FORBIDDEN: 'Insufficient API key scope',
+    NOT_FOUND: 'Resource does not exist',
+    VALIDATION_ERROR: 'Request validation failed',
+    PAYLOAD_TOO_LARGE: 'Request body is too large',
+    RATE_LIMITED: 'Too many requests, please try again later',
+    SERVICE_UNAVAILABLE: 'Service temporarily unavailable',
+    INTERNAL_ERROR: 'An unexpected error occurred',
+    WEBHOOK_LIMIT_EXCEEDED: 'Webhook limit exceeded for this subscriber',
+    WEBHOOK_NOT_FOUND: 'Webhook not found',
+    AIRDROP_NOT_FOUND: 'Airdrop not found',
+  },
+  es: {
+    UNAUTHORIZED: 'Clave de API faltante o inválida',
+    FORBIDDEN: 'Alcance de clave de API insuficiente',
+    NOT_FOUND: 'El recurso no existe',
+    VALIDATION_ERROR: 'Validación de solicitud fallida',
+    PAYLOAD_TOO_LARGE: 'El cuerpo de la solicitud es demasiado grande',
+    RATE_LIMITED: 'Demasiadas solicitudes, intente más tarde',
+    SERVICE_UNAVAILABLE: 'Servicio temporalmente no disponible',
+    INTERNAL_ERROR: 'Ocurrió un error inesperado',
+    WEBHOOK_LIMIT_EXCEEDED: 'Límite de webhook excedido para este suscriptor',
+    WEBHOOK_NOT_FOUND: 'Webhook no encontrado',
+    AIRDROP_NOT_FOUND: 'Airdrop no encontrado',
+  },
+  fr: {
+    UNAUTHORIZED: 'Clé API manquante ou invalide',
+    FORBIDDEN: 'Portée de clé API insuffisante',
+    NOT_FOUND: 'La ressource n\'existe pas',
+    VALIDATION_ERROR: 'Validation de la demande échouée',
+    PAYLOAD_TOO_LARGE: 'Le corps de la demande est trop volumineux',
+    RATE_LIMITED: 'Trop de demandes, réessayez plus tard',
+    SERVICE_UNAVAILABLE: 'Service temporairement indisponible',
+    INTERNAL_ERROR: 'Une erreur inattendue s\'est produite',
+    WEBHOOK_LIMIT_EXCEEDED: 'Limite de webhook dépassée pour cet abonné',
+    WEBHOOK_NOT_FOUND: 'Webhook non trouvé',
+    AIRDROP_NOT_FOUND: 'Airdrop non trouvé',
+  },
+};
+
+function parseAcceptLanguage(header) {
+  if (!header) return 'en';
+  const parts = header.split(',').map((part) => {
+    const [lang, q] = part.trim().split(';');
+    const quality = q ? parseFloat(q.split('=')[1]) : 1.0;
+    return { lang: lang.trim().split('-')[0].toLowerCase(), quality };
+  });
+  parts.sort((a, b) => b.quality - a.quality);
+  return parts.length > 0 ? parts[0].lang : 'en';
+}
+
+function getLocale(req) {
+  const acceptLanguage = req.get('accept-language');
+  const preferred = parseAcceptLanguage(acceptLanguage);
+  return Object.prototype.hasOwnProperty.call(translations, preferred) ? preferred : 'en';
+}
+
+function translate(code, locale = 'en') {
+  const localeTranslations = translations[locale] || translations.en;
+  return localeTranslations[code] || translations.en[code] || code;
+}
+
+module.exports = {
+  translate,
+  getLocale,
+  translations,
+};
diff --git a/src/services/webhookDispatcher.js b/src/services/webhookDispatcher.js
index 9a29fbe..4a7815d 100644
--- a/src/services/webhookDispatcher.js
+++ b/src/services/webhookDispatcher.js
@@ -436,7 +436,12 @@ async function attempt(deliveryId, sequence) {
       const res = await postOnce(webhook.url, headers, body, webhook.timeoutMs);
       responseStatus = res.status;
     } catch (err) {
-      networkError = err.message || "network error";
+      const errorCode = err?.code || err?.errno;
+      if (errorCode === 'ETIMEDOUT') {
+        networkError = 'webhook request timeout';
+      } else {
+        networkError = err.message || "network error";
+      }
     }
 
     const succeeded =