diff --git a/apps/web/src/app/api/locale/route.ts b/apps/web/src/app/api/locale/route.ts
new file mode 100644
index 0000000..179e9b3
--- /dev/null
+++ b/apps/web/src/app/api/locale/route.ts
@@ -0,0 +1,16 @@
+import { NextRequest, NextResponse } from 'next/server'
+import { SUPPORTED_LOCALES, type Locale } from '@/middleware'
+
+/**
+ * POST /api/locale  { locale: "es" }
+ * Sets the locale cookie so subsequent requests use the chosen locale.
+ */
+export async function POST(req: NextRequest) {
+  const { locale } = await req.json().catch(() => ({}))
+  if (!SUPPORTED_LOCALES.includes(locale as Locale)) {
+    return NextResponse.json({ error: 'Unsupported locale' }, { status: 400 })
+  }
+  const res = NextResponse.json({ locale })
+  res.cookies.set('locale', locale as string, { path: '/', sameSite: 'lax', maxAge: 60 * 60 * 24 * 365 })
+  return res
+}
diff --git a/apps/web/src/app/layout.tsx b/apps/web/src/app/layout.tsx
index df2c11f..5ae093e 100644
--- a/apps/web/src/app/layout.tsx
+++ b/apps/web/src/app/layout.tsx
@@ -1,6 +1,7 @@
 // #481 unsupported-browser landing page.
 import type { Metadata } from 'next'
 import { Inter } from 'next/font/google'
+import { headers } from 'next/headers'
 import './globals.css'
 import { Providers } from './providers'
 import { Navbar } from '@/components/navbar'
diff --git a/apps/web/src/lib/i18n.ts b/apps/web/src/lib/i18n.ts
new file mode 100644
index 0000000..0ff26ab
--- /dev/null
+++ b/apps/web/src/lib/i18n.ts
@@ -0,0 +1,94 @@
+import type { Locale } from '../middleware'
+
+// ---------------------------------------------------------------------------
+// Message catalogue — add keys here, translate in each locale object
+// ---------------------------------------------------------------------------
+const messages = {
+  en: {
+    'home.title': 'SolarProof',
+    'home.tagline': 'End-to-end cryptographic proof of renewable energy on Stellar.',
+    'home.subtitle': 'Every kWh signed at the meter · anchored on-chain · publicly verifiable',
+    'home.verify_cta': 'Verify a Certificate',
+    'home.dashboard_cta': 'Dashboard',
+    'home.chain_title': 'Chain of custody',
+    'nav.verify': 'Verify',
+    'nav.dashboard': 'Dashboard',
+    'verify.title': 'Verify Certificate',
+    'verify.placeholder': 'Certificate ID or transaction hash',
+    'verify.submit': 'Verify',
+    'error.not_found': 'Not found',
+    'error.invalid_signature': 'Invalid signature',
+  },
+  es: {
+    'home.title': 'SolarProof',
+    'home.tagline': 'Prueba criptográfica de extremo a extremo de energía renovable en Stellar.',
+    'home.subtitle': 'Cada kWh firmado en el medidor · anclado en cadena · verificable públicamente',
+    'home.verify_cta': 'Verificar certificado',
+    'home.dashboard_cta': 'Panel',
+    'home.chain_title': 'Cadena de custodia',
+    'nav.verify': 'Verificar',
+    'nav.dashboard': 'Panel',
+    'verify.title': 'Verificar certificado',
+    'verify.placeholder': 'ID de certificado o hash de transacción',
+    'verify.submit': 'Verificar',
+    'error.not_found': 'No encontrado',
+    'error.invalid_signature': 'Firma no válida',
+  },
+  fr: {
+    'home.title': 'SolarProof',
+    'home.tagline': 'Preuve cryptographique de bout en bout de l\'énergie renouvelable sur Stellar.',
+    'home.subtitle': 'Chaque kWh signé au compteur · ancré on-chain · vérifiable publiquement',
+    'home.verify_cta': 'Vérifier un certificat',
+    'home.dashboard_cta': 'Tableau de bord',
+    'home.chain_title': 'Chaîne de custody',
+    'nav.verify': 'Vérifier',
+    'nav.dashboard': 'Tableau de bord',
+    'verify.title': 'Vérifier le certificat',
+    'verify.placeholder': 'ID de certificat ou hash de transaction',
+    'verify.submit': 'Vérifier',
+    'error.not_found': 'Introuvable',
+    'error.invalid_signature': 'Signature invalide',
+  },
+  de: {
+    'home.title': 'SolarProof',
+    'home.tagline': 'Ende-zu-Ende-kryptografischer Nachweis erneuerbarer Energie auf Stellar.',
+    'home.subtitle': 'Jede kWh am Zähler signiert · on-chain verankert · öffentlich prüfbar',
+    'home.verify_cta': 'Zertifikat prüfen',
+    'home.dashboard_cta': 'Dashboard',
+    'home.chain_title': 'Herkunftsnachweis',
+    'nav.verify': 'Prüfen',
+    'nav.dashboard': 'Dashboard',
+    'verify.title': 'Zertifikat prüfen',
+    'verify.placeholder': 'Zertifikats-ID oder Transaktions-Hash',
+    'verify.submit': 'Prüfen',
+    'error.not_found': 'Nicht gefunden',
+    'error.invalid_signature': 'Ungültige Signatur',
+  },
+  pt: {
+    'home.title': 'SolarProof',
+    'home.tagline': 'Prova criptográfica ponta a ponta de energia renovável no Stellar.',
+    'home.subtitle': 'Cada kWh assinado no medidor · ancorado on-chain · verificável publicamente',
+    'home.verify_cta': 'Verificar certificado',
+    'home.dashboard_cta': 'Painel',
+    'home.chain_title': 'Cadeia de custódia',
+    'nav.verify': 'Verificar',
+    'nav.dashboard': 'Painel',
+    'verify.title': 'Verificar certificado',
+    'verify.placeholder': 'ID de certificado ou hash de transação',
+    'verify.submit': 'Verificar',
+    'error.not_found': 'Não encontrado',
+    'error.invalid_signature': 'Assinatura inválida',
+  },
+} satisfies Record<Locale, Record<string, string>>
+
+export type MessageKey = keyof typeof messages['en']
+
+/**
+ * Returns a translate function for the given locale.
+ * Falls back to English if the key is missing in the requested locale.
+ */
+export function getTranslations(locale: Locale) {
+  return function t(key: MessageKey): string {
+    return messages[locale]?.[key] ?? messages['en'][key] ?? key
+  }
+}