diff --git a/README.md b/README.md
index aaf34c3..c6e165a 100644
--- a/README.md
+++ b/README.md
@@ -62,6 +62,41 @@ export const coreFetch = createCoreFetch({
 });
 ```
 
-## Roadmap
+## i18n (v0.3)
 
-- **v0.3** — i18n system (de/en × formal/informal with fallback chain).
+Two axes: **locale** (`de`, `en`, …) × **tone** (`formal`, `informal`).
+Bundles are keyed by `<locale>` (fallback) and `<locale>-<tone>` (primary);
+the resolver walks `<cur-locale>-<cur-tone>` → `<cur-locale>` →
+`<default-locale>-<default-tone>` → `<default-locale>` → the raw key
+(missing keys log via an `onMiss` hook instead of breaking render).
+
+```ts
+// Define bundles — only strings that differ between tones need a
+// tone-specific entry. Everything else lives in the locale bundle.
+import { createI18n }    from '@uschuster/webapp-scaffold/i18n';
+import { useI18nStore }  from '@uschuster/webapp-scaffold/i18n-react';  // React-only
+
+const bundles = {
+    'de':          { greeting: 'Willkommen',  see_more: 'Mehr anzeigen' },
+    'de-informal': { greeting: 'Hi!' },
+    'en':          { greeting: 'Welcome',     see_more: 'See more' },
+} as const;
+
+export const i18n = createI18n<typeof bundles['de']>({
+    bundles,
+    defaultLocale: 'de',
+    defaultTone:   'formal',
+    onMiss: (key, loc, tone) =>
+        console.warn(`[i18n] missing ${String(key)} @ ${loc}-${tone}`),
+});
+
+// React — components re-render on setLocale / setTone:
+export const useI18n = () => useI18nStore(i18n);
+
+// In a component:
+const { t, setTone } = useI18n();
+return <button onClick={() => setTone('informal')}>{t('greeting')}</button>;
+```
+
+SSR-safe: pass `initialLocale` / `initialTone` from the server render.
+Tree-shakeable: only the bundles you import ship in the bundle.
diff --git a/package.json b/package.json
index ad1bc13..5db8dc8 100644
--- a/package.json
+++ b/package.json
@@ -1,6 +1,6 @@
 {
   "name": "@uschuster/webapp-scaffold",
-  "version": "0.2.0",
+  "version": "0.3.0",
   "description": "Shared build scripts + Vite config factories for webapp-template-derived projects.",
   "type": "module",
   "bin": {
@@ -10,14 +10,20 @@
   },
   "main": "src/vite-config.ts",
   "exports": {
-    ".":              "./src/vite-config.ts",
-    "./core-fetch":   "./src/core-fetch.ts",
+    ".":                "./src/vite-config.ts",
+    "./core-fetch":     "./src/core-fetch.ts",
+    "./i18n":           "./src/i18n.ts",
+    "./i18n-react":     "./src/i18n-react.ts",
     "./orval-template": "./templates/orval.config.template.ts"
   },
   "files": ["bin/", "src/", "templates/", "README.md", "LICENSE"],
   "peerDependencies": {
     "vite":              "^6.0.0",
-    "@vitejs/plugin-react": "^4.0.0"
+    "@vitejs/plugin-react": "^4.0.0",
+    "react":             "^19.0.0"
+  },
+  "peerDependenciesMeta": {
+    "react": { "optional": true }
   },
   "repository": {
     "type": "git",
diff --git a/src/i18n-react.ts b/src/i18n-react.ts
new file mode 100644
index 0000000..bda74b4
--- /dev/null
+++ b/src/i18n-react.ts
@@ -0,0 +1,17 @@
+// React bindings for the i18n store — kept separate so `src/i18n.ts` has
+// no React peer dependency. Import from `@uschuster/webapp-scaffold/i18n-react`.
+
+import { useSyncExternalStore } from 'react';
+import type { Bundle, I18n } from './i18n.js';
+
+/**
+ * Subscribe a component to locale/tone changes. Returns the same `i18n`
+ * store you passed in — `t`, `setLocale`, etc. are stable across renders.
+ *
+ *     const i18n = createI18n({ ... });
+ *     export const useI18n = () => useI18nStore(i18n);
+ */
+export function useI18nStore<B extends Bundle>(i18n: I18n<B>) {
+    useSyncExternalStore(i18n.subscribe, i18n.getSnapshot, i18n.getSnapshot);
+    return i18n;
+}
diff --git a/src/i18n.ts b/src/i18n.ts
new file mode 100644
index 0000000..1af96d0
--- /dev/null
+++ b/src/i18n.ts
@@ -0,0 +1,92 @@
+// i18n for webapp-template-derived apps.
+//
+// Axes: locale ('de' | 'en') × tone ('formal' | 'informal').
+// A translation bundle is a map of keys → strings. Consumers provide
+// bundles keyed by `locale` (fallback) and `locale-tone` (primary),
+// e.g. { 'de': ..., 'de-informal': ..., 'en': ... }. The resolver walks:
+//
+//     <cur-locale>-<cur-tone>
+//  → <cur-locale>
+//  → <default-locale>-<default-tone>
+//  → <default-locale>
+//  → raw key (so missing keys log instead of breaking rendering)
+//
+// Tree-shakeable: consumers import only the bundles they ship. SSR-safe:
+// the store starts from whatever locale/tone the server passes, no
+// window access at construction time. React bits live in a separate
+// entry — `@uschuster/webapp-scaffold/i18n-react` — so this file has no
+// React peer dependency.
+
+export type Locale = string;                 // e.g. 'de', 'en'
+export type Tone   = 'formal' | 'informal';
+
+export type Bundle = Record<string, string>;
+
+/**
+ * Partial bundles per `<locale>` or `<locale>-<tone>` key. Only the
+ * defaultLocale bundle is required to be complete at runtime — others
+ * fall through to it. The type is Partial so tone overrides only need
+ * the strings that actually differ from the tone-less locale bundle.
+ */
+export type I18nBundles<B extends Bundle> = Record<string, Partial<B>>;
+
+export interface CreateI18nOptions<B extends Bundle> {
+    bundles:        I18nBundles<B>;
+    defaultLocale:  Locale;
+    defaultTone?:   Tone;
+    initialLocale?: Locale;
+    initialTone?:   Tone;
+    /** Called once for each first-time miss, to surface gaps during dev. */
+    onMiss?:        (key: keyof B, locale: Locale, tone: Tone) => void;
+}
+
+export interface I18n<B extends Bundle> {
+    t:            (key: keyof B) => string;
+    locale:       () => Locale;
+    tone:         () => Tone;
+    setLocale:    (l: Locale) => void;
+    setTone:      (t: Tone) => void;
+    subscribe:    (fn: () => void) => () => void;
+    getSnapshot:  () => string;
+}
+
+export function createI18n<B extends Bundle>(opts: CreateI18nOptions<B>): I18n<B> {
+    const defTone: Tone = opts.defaultTone ?? 'formal';
+    let locale: Locale = opts.initialLocale ?? opts.defaultLocale;
+    let tone:   Tone   = opts.initialTone   ?? defTone;
+    const missed = new Set<string>();
+    const listeners = new Set<() => void>();
+    const notify = () => listeners.forEach(fn => fn());
+
+    const chain = (): string[] => [
+        `${locale}-${tone}`,
+        locale,
+        `${opts.defaultLocale}-${defTone}`,
+        opts.defaultLocale,
+    ];
+
+    function t(key: keyof B): string {
+        for (const b of chain()) {
+            const bundle = opts.bundles[b];
+            if (bundle && (key as string) in bundle) {
+                return (bundle as B)[key as string];
+            }
+        }
+        const miss = `${String(key)}@${locale}-${tone}`;
+        if (!missed.has(miss)) {
+            missed.add(miss);
+            opts.onMiss?.(key, locale, tone);
+        }
+        return String(key);
+    }
+
+    return {
+        t,
+        locale: () => locale,
+        tone:   () => tone,
+        setLocale: (l) => { if (l !== locale) { locale = l; notify(); } },
+        setTone:   (t) => { if (t !== tone)   { tone   = t; notify(); } },
+        subscribe: (fn) => { listeners.add(fn); return () => { listeners.delete(fn); }; },
+        getSnapshot: () => `${locale}-${tone}`,
+    };
+}