Index: src/services/i18n.js
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--- src/services/i18n.js	(revision 03fa5481c4f9961f422ed9a06824e8944fb868bf)
+++ src/services/i18n.js	(revision 03fa5481c4f9961f422ed9a06824e8944fb868bf)
@@ -0,0 +1,117 @@
+// i18n — eenvoudige interface-vertaling (UI-strings), bezoeker-instelbaar.
+//
+// Taalkeuze: req.session.lang (gezet via /lang/:code) → anders browser-taal
+// (Accept-Language) → anders 'nl'. De helper t(lang, key, vars) zoekt de string op
+// in DICT[lang], valt terug op 'nl', dan op de key zelf. Alleen INTERFACE-teksten;
+// door gebruikers geschreven content (posts, sitenaam, bio) wordt niet vertaald.
+
+export const SUPPORTED = ['nl', 'en', 'de'];
+export const LANG_NAMES = { nl: 'Nederlands', en: 'English', de: 'Deutsch' };
+
+const DICT = {
+  nl: {
+    'nav.back_to_site': '← Terug naar site',
+    'nav.home': 'Home',
+    'nav.archive': 'Archief',
+    'nav.search': 'Zoeken',
+    'nav.theme': 'Thema wisselen',
+    'nav.install': 'App installeren',
+    'nav.login': 'Inloggen',
+    'nav.logout': 'Uitloggen',
+    'nav.admin': 'Beheer',
+    'nav.account': 'Account',
+    'nav.profile': 'Profiel',
+    'nav.favorites': 'Favorieten',
+    'nav.new_post': 'Nieuwe post',
+    'nav.language': 'Taal',
+    'switch.agenda': 'Agenda',
+    'switch.solo': 'Solo',
+    'switch.circle': 'Cirkel',
+    'switch.timeline': 'Tijdlijn',
+    'switch.grid': 'Grid',
+    'postnav.newer': 'Nieuwer',
+    'postnav.older': 'Ouder',
+    'postnav.newest': 'Nieuwste post',
+    'postnav.oldest': 'Oudste post',
+    'footer.subscribe_cta': 'Blijf op de hoogte',
+    'footer.subscribe': 'Inschrijven',
+    'footer.install': 'Installeer app',
+    'common.email_placeholder': 'jouw@email.nl',
+  },
+  en: {
+    'nav.back_to_site': '← Back to site',
+    'nav.home': 'Home',
+    'nav.archive': 'Archive',
+    'nav.search': 'Search',
+    'nav.theme': 'Toggle theme',
+    'nav.install': 'Install app',
+    'nav.login': 'Log in',
+    'nav.logout': 'Log out',
+    'nav.admin': 'Admin',
+    'nav.account': 'Account',
+    'nav.profile': 'Profile',
+    'nav.favorites': 'Favorites',
+    'nav.new_post': 'New post',
+    'nav.language': 'Language',
+    'switch.agenda': 'Agenda',
+    'switch.solo': 'Solo',
+    'switch.circle': 'Circle',
+    'switch.timeline': 'Timeline',
+    'switch.grid': 'Grid',
+    'postnav.newer': 'Newer',
+    'postnav.older': 'Older',
+    'postnav.newest': 'Newest post',
+    'postnav.oldest': 'Oldest post',
+    'footer.subscribe_cta': 'Stay in the loop',
+    'footer.subscribe': 'Subscribe',
+    'footer.install': 'Install app',
+    'common.email_placeholder': 'you@email.com',
+  },
+  de: {
+    'nav.back_to_site': '← Zurück zur Seite',
+    'nav.home': 'Start',
+    'nav.archive': 'Archiv',
+    'nav.search': 'Suche',
+    'nav.theme': 'Thema wechseln',
+    'nav.install': 'App installieren',
+    'nav.login': 'Anmelden',
+    'nav.logout': 'Abmelden',
+    'nav.admin': 'Verwaltung',
+    'nav.account': 'Konto',
+    'nav.profile': 'Profil',
+    'nav.favorites': 'Favoriten',
+    'nav.new_post': 'Neuer Beitrag',
+    'nav.language': 'Sprache',
+    'switch.agenda': 'Termine',
+    'switch.solo': 'Solo',
+    'switch.circle': 'Kreis',
+    'switch.timeline': 'Zeitleiste',
+    'switch.grid': 'Raster',
+    'postnav.newer': 'Neuer',
+    'postnav.older': 'Älter',
+    'postnav.newest': 'Neuester Beitrag',
+    'postnav.oldest': 'Ältester Beitrag',
+    'footer.subscribe_cta': 'Bleib auf dem Laufenden',
+    'footer.subscribe': 'Abonnieren',
+    'footer.install': 'App installieren',
+    'common.email_placeholder': 'du@email.de',
+  },
+};
+
+export function t(lang, key, vars) {
+  const l = SUPPORTED.includes(lang) ? lang : 'nl';
+  let s = (DICT[l] && DICT[l][key]);
+  if (s === undefined) s = (DICT.nl[key] !== undefined ? DICT.nl[key] : key);
+  if (vars) for (const k in vars) s = s.replace(new RegExp('\\{' + k + '\\}', 'g'), vars[k]);
+  return s;
+}
+
+// Bepaal de taal voor dit request: sessie-keuze → browser-taal → nl.
+export function resolveLang(req) {
+  const s = req && req.session && req.session.lang;
+  if (s && SUPPORTED.includes(s)) return s;
+  const al = ((req && req.headers && req.headers['accept-language']) || '').toLowerCase();
+  const first = al.split(',')[0].trim().slice(0, 2);
+  if (SUPPORTED.includes(first)) return first;
+  return 'nl';
+}