diff --git a/CHANGELOG.md b/CHANGELOG.md
index b02389c..7d7691d 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -7,6 +7,17 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ## [Unreleased]
 
+## [0.5.0] - 2026-03-31
+
+### Added
+- i18n: full internationalisation system (`public/i18n.js`) with German (de) and English (en) support; language auto-detected from `navigator.language`, overridable via Settings
+- i18n: all user-facing strings moved to locale files (`public/locales/de.json`, `public/locales/en.json`); 489 translation keys covering all modules
+- i18n: locale switch without page reload — all pages, components and navigation re-render via `locale-changed` custom event
+- i18n: `oikos-locale-picker` Web Component in Settings — three options: System (follows browser language), Deutsch, English
+- i18n: dates and times formatted with `Intl.DateTimeFormat` using the active locale; `formatDate()` and `formatTime()` exported from `i18n.js`
+- i18n: fallback chain (active locale → German → key) ensures no untranslated keys are shown even if a future locale file is incomplete
+- i18n: adding a new language requires only one JSON file (`public/locales/xx.json`) and one line in `SUPPORTED_LOCALES`
+
 ## [0.4.0] - 2026-03-31
 
 ### Fixed
@@ -101,7 +112,8 @@ Initial release of Oikos — a self-hosted family planner for 2–6 person house
 - No user data cached by service worker (API requests are network-only)
 - Hardened `.gitignore` and `.dockerignore` to prevent accidental secret or binary leakage
 
-[Unreleased]: https://github.com/ulsklyc/oikos/compare/v0.4.0...HEAD
+[Unreleased]: https://github.com/ulsklyc/oikos/compare/v0.5.0...HEAD
+[0.5.0]: https://github.com/ulsklyc/oikos/compare/v0.4.0...v0.5.0
 [0.4.0]: https://github.com/ulsklyc/oikos/compare/v0.3.0...v0.4.0
 [0.3.0]: https://github.com/ulsklyc/oikos/compare/v0.2.1...v0.3.0
 [0.2.1]: https://github.com/ulsklyc/oikos/compare/v0.1.0...v0.2.1
diff --git a/docs/SPEC.md b/docs/SPEC.md
index b2512f3..8b79a73 100644
--- a/docs/SPEC.md
+++ b/docs/SPEC.md
@@ -232,6 +232,7 @@ Benutzerverwaltung und App-Konfiguration. Nur für eingeloggte Nutzer.
 - **Benutzerverwaltung (Admin):** Neue Benutzer anlegen, bestehende Benutzer bearbeiten/löschen, Rollen zuweisen (admin/member)
 - **Kalender-Integration:** Google Calendar OAuth verbinden/trennen, Apple Calendar (CalDAV) Credentials hinterlegen, Sync-Intervall konfigurieren
 - **Wetter:** OpenWeatherMap Standort konfigurieren
+- **Sprache:** System (folgt `navigator.language`), Deutsch, English — via `oikos-locale-picker` Web Component; Wechsel ohne Reload
 - **App-Info:** Version, Lizenz
 
 ### Budget (`/budget`)
@@ -313,3 +314,33 @@ Benutzerverwaltung und App-Konfiguration. Nur für eingeloggte Nutzer.
 - Mobil: < 768px (1 Spalte, Bottom Nav)
 - Tablet: 768–1024px (2 Spalten, Bottom Nav)
 - Desktop: > 1024px (Sidebar + Content)
+
+---
+
+## Internationalisierung (i18n)
+
+Alle UI-Strings werden über `public/i18n.js` verwaltet. Kein hardcodierter Text in JS-Dateien außer in Locale-Dateien.
+
+### Architektur
+
+- **Modul:** `public/i18n.js` — exports: `initI18n()`, `setLocale()`, `t(key, params?)`, `getLocale()`, `getSupportedLocales()`, `formatDate(date)`, `formatTime(date)`
+- **Locale-Dateien:** `public/locales/de.json` (Referenz), `public/locales/en.json` — Struktur: `{ "modul.camelCaseKey": "Wert" }`
+- **Variablen:** `{{variable}}`-Syntax in Übersetzungsstrings, z.B. `t('tasks.assignedTo', { name: 'Anna' })`
+- **Fallback-Kette:** aktive Locale → Deutsch (`de`) → Key selbst
+- **Datumsformat:** `Intl.DateTimeFormat` mit aktuellem Locale — `formatDate()` und `formatTime()` aus `i18n.js`
+
+### Sprach-Erkennung
+
+1. `localStorage` Eintrag `oikos-locale` (manuelle Auswahl)
+2. `navigator.languages[0]` (Browser-Sprache)
+3. Fallback: `de`
+
+### Neue Sprache hinzufügen
+
+1. `public/locales/xx.json` erstellen (Kopie von `de.json`, übersetzen)
+2. `SUPPORTED_LOCALES` in `public/i18n.js` um `'xx'` erweitern
+3. Label in `oikos-locale-picker` ergänzen (`LOCALE_LABELS['xx'] = 'Name'`)
+
+### Locale-Wechsel
+
+`setLocale(locale)` speichert die Auswahl, lädt die neue Locale-Datei und feuert das `locale-changed` Custom Event. Alle Seiten-Module und Web Components hören dieses Event und rendern sich neu — kein Seiten-Reload nötig.
diff --git a/package-lock.json b/package-lock.json
index ac78d07..e819a38 100644
--- a/package-lock.json
+++ b/package-lock.json
@@ -1,12 +1,12 @@
 {
   "name": "oikos",
-  "version": "0.3.0",
+  "version": "0.5.0",
   "lockfileVersion": 3,
   "requires": true,
   "packages": {
     "": {
       "name": "oikos",
-      "version": "0.3.0",
+      "version": "0.5.0",
       "license": "MIT",
       "dependencies": {
         "bcrypt": "^5.1.1",
diff --git a/package.json b/package.json
index 6760a40..7fadc83 100644
--- a/package.json
+++ b/package.json
@@ -1,6 +1,6 @@
 {
   "name": "oikos",
-  "version": "0.3.0",
+  "version": "0.5.0",
   "description": "Selbstgehosteter Familienplaner — Kalender, Aufgaben, Einkauf, Essensplan, Budget und mehr. Privat, offen, ohne Abo.",
   "main": "server/index.js",
   "scripts": {