diff --git a/frontend/src/components/LayoutBar.tsx b/frontend/src/components/LayoutBar.tsx
index a335836..768f45d 100644
--- a/frontend/src/components/LayoutBar.tsx
+++ b/frontend/src/components/LayoutBar.tsx
@@ -11,6 +11,7 @@ import type { LayoutDefinition } from '../types/cad.types';
 import { getActiveDocument, onActiveDocumentChanged } from '../kernel/document/documentService';
 import { insertTitleBlockForLayout } from '../services/titleBlockService';
 import { printLayout } from '../services/printService';
+import { useT } from '../i18n/strings';
 
 export interface LayoutView {
   center: { x: number; y: number };
@@ -80,6 +81,8 @@ const WIDGET_W = 100;
 const WIDGET_H = 80;
 
 const LayoutBar: React.FC<LayoutBarProps> = ({ doc: docProp, bridge: bridgeProp }) => {
+  const t = useT();
+
   const [doc, setDoc] = useState(docProp ?? getActiveDocument());
   const [layouts, setLayouts] = useState<LayoutDefinition[]>([]);
 
@@ -134,23 +137,23 @@ const LayoutBar: React.FC<LayoutBarProps> = ({ doc: docProp, bridge: bridgeProp
   return (
     <div className="layout-bar" data-testid="layout-bar">
       <div className="layout-bar-header">
-        <span className="layout-bar-title">Layouts</span>
+        <span className="layout-bar-title">{t('layouts.title')}</span>
         <button
           type="button"
           className="layout-bar-add"
-          title="Layout hinzufügen"
+          title={t('layouts.add')}
           onClick={handleAdd}
           disabled={!bridge || !doc}
         >+</button>
         <button
           type="button"
           className="layout-bar-print screen-only"
-          title="Layout drucken (A4 landscape)"
+          title={t('layouts.print')}
           onClick={handlePrint}
         >🖨</button>
       </div>
       {layouts.length === 0 ? (
-        <div className="layout-bar-empty">Keine Layouts</div>
+        <div className="layout-bar-empty">{t('layouts.empty')}</div>
       ) : (
         <div className="layout-bar-list">
           {layouts.map((l) => {
@@ -161,7 +164,7 @@ const LayoutBar: React.FC<LayoutBarProps> = ({ doc: docProp, bridge: bridgeProp
                   type="button"
                   className="layout-bar-entry"
                   data-testid={`layout-entry-${l.id}`}
-                  title={`Zoom zu ${l.name}`}
+                  title={t('layouts.zoomto', l.name)}
                   onClick={() => handleZoomTo(l)}
                 >
                   <svg
@@ -187,7 +190,7 @@ const LayoutBar: React.FC<LayoutBarProps> = ({ doc: docProp, bridge: bridgeProp
                 <button
                   type="button"
                   className="layout-bar-tb"
-                  title={`Titelblock für ${l.name} einfügen/aktualisieren`}
+                  title={t('layouts.titleblock', l.name)}
                   onClick={() => {
                     if (!doc) return;
                     insertTitleBlockForLayout(doc as never, l.id, {
@@ -202,7 +205,7 @@ const LayoutBar: React.FC<LayoutBarProps> = ({ doc: docProp, bridge: bridgeProp
                 <button
                   type="button"
                   className="layout-bar-del"
-                  title={`Layout ${l.name} löschen`}
+                  title={t('layouts.delete', l.name)}
                   onClick={() => handleDelete(l.id)}
                 >🗑</button>
               </div>
diff --git a/frontend/src/components/Topbar.tsx b/frontend/src/components/Topbar.tsx
index b67416c..197f88f 100644
--- a/frontend/src/components/Topbar.tsx
+++ b/frontend/src/components/Topbar.tsx
@@ -2,12 +2,15 @@ import React from 'react';
 import type { TopbarProps } from '../types/ui.types';
 import type { UnitType } from '../utils/format';
 import { useUIMode, toggleUIMode, getUIMode } from '../services/uiModeService';
+import { useT } from '../i18n/strings';
 
 const Topbar: React.FC<TopbarProps> = ({
   projectName, savedStatus, onUndo, onRedo, onThemeToggle, theme,
   onOpenSettings, onOpenLeftDrawer, onOpenRightDrawer,
   unit = 'mm', onUnitChange, onNavigateBack,
 }) => {
+  const t = useT();
+
   const uiMode = useUIMode();
 
   return (
@@ -38,8 +41,8 @@ const Topbar: React.FC<TopbarProps> = ({
       <div className="topbar-right">
         <button
           className="icon-btn-top"
-          title={uiMode === 'pro' ? 'Zum einfachen Modus wechseln' : 'Zum Profi-Modus wechseln'}
-          aria-label={uiMode === 'pro' ? 'Zum einfachen Modus wechseln' : 'Zum Profi-Modus wechseln'}
+          title={uiMode === 'pro' ? t('mode.toSimple') : t('mode.toPro')}
+          aria-label={uiMode === 'pro' ? t('mode.toSimple') : t('mode.toPro')}
           onClick={toggleUIMode}
         >
           {uiMode === 'pro' ? '😊' : '🛠'}
diff --git a/frontend/src/i18n/strings.ts b/frontend/src/i18n/strings.ts
new file mode 100644
index 0000000..3cdf605
--- /dev/null
+++ b/frontend/src/i18n/strings.ts
@@ -0,0 +1,105 @@
+/**
+ * Task H4 – i18n-Grundlage: zentrale Strings-Maps de/en.
+ *
+ * t(key) liefert den Text der aktuellen Sprache; unbekannte Keys
+ * fallen auf den Key zurück (im Code sichtbar, lint-bar über Tests).
+ * Sprache persistiert in localStorage ('webcad.language', Default de).
+ * Komponenten nutzen useT() für Live-Updates bei Sprachwechsel.
+ *
+ * Vollständige mechanische Ersetzung ALLER Komponenten-Texte ist
+ * teilautomatisierbares Follow-up; hier liegt die Infrastruktur und
+ * die Kern-UI-Texte (LayoutBar, Modus).
+ */
+import { useEffect, useState } from 'react';
+
+export const LANGUAGES = ['de', 'en'] as const;
+export type Language = (typeof LANGUAGES)[number];
+
+type StringEntry = string | ((arg?: string) => string);
+
+const STORAGE_KEY = 'webcad.language';
+
+/** Zentrale Text-Maps (Key-Pfad → Text oder parametrisierte Funktion). */
+const DE: Record<string, StringEntry> = {
+  'layouts.title': 'Layouts',
+  'layouts.add': 'Layout hinzufügen',
+  'layouts.empty': 'Keine Layouts',
+  'layouts.delete': (n?: string) => 'Layout ' + (n ?? '') + ' löschen',
+  'layouts.print': 'Layout drucken (A4 landscape)',
+  'layouts.titleblock': (n?: string) => 'Titelblock für ' + (n ?? '') + ' einfügen/aktualisieren',
+  'layouts.zoomto': (n?: string) => 'Zoom zu ' + (n ?? ''),
+  'mode.toSimple': 'Zum einfachen Modus wechseln',
+  'mode.toPro': 'Zum Profi-Modus wechseln',
+};
+
+const EN: Record<string, StringEntry> = {
+  'layouts.title': 'Layouts',
+  'layouts.add': 'Add layout',
+  'layouts.empty': 'No layouts',
+  'layouts.delete': (n?: string) => 'Delete layout ' + (n ?? ''),
+  'layouts.print': 'Print layout (A4 landscape)',
+  'layouts.titleblock': (n?: string) => 'Insert/update title block for ' + (n ?? ''),
+  'layouts.zoomto': (n?: string) => 'Zoom to ' + (n ?? ''),
+  'mode.toSimple': 'Switch to simple mode',
+  'mode.toPro': 'Switch to pro mode',
+};
+
+function mapFor(lang: Language): Record<string, StringEntry> {
+  return lang === 'en' ? EN : DE;
+}
+
+function readStored(): Language {
+  try {
+    const raw = localStorage.getItem(STORAGE_KEY);
+    if (raw === 'de' || raw === 'en') return raw;
+  } catch {
+    // localStorage nicht verfügbar
+  }
+  return 'de';
+}
+
+let current: Language = readStored();
+const listeners = new Set<(lang: Language) => void>();
+
+/** Aktuelle Sprache. */
+export function getLanguage(): Language {
+  return current;
+}
+
+/** Sprache wechseln und persistieren. */
+export function setLanguage(lang: Language): void {
+  current = lang;
+  try {
+    localStorage.setItem(STORAGE_KEY, lang);
+  } catch {
+    // Persistenz optional
+  }
+  for (const cb of listeners) cb(lang);
+}
+
+/** Subscribe auf Sprachwechsel; gibt Unsubscribe zurück. */
+export function subscribeLanguage(cb: (lang: Language) => void): () => void {
+  listeners.add(cb);
+  return () => { listeners.delete(cb); };
+}
+
+/**
+ * Übersetzung: t('key') bzw. t('key', arg) für parametrisierte Texte.
+ * Unbekannter Key → Key als Fallback (offensichtlich sichtbar).
+ */
+export function t(key: string, arg?: string): string {
+  const entry = mapFor(current)[key];
+  if (typeof entry === 'function') return entry(arg);
+  if (typeof entry === 'string') return entry;
+  return key;
+}
+
+/** React-Hook: erzwingt Re-Render bei Sprachwechsel. */
+export function useT(): typeof t {
+  const [, force] = useState(0);
+  useEffect(() => {
+    const off = subscribeLanguage(() => force((n) => n + 1));
+    return off;
+  }, []);
+  return t;
+}
diff --git a/frontend/tests/i18n.test.tsx b/frontend/tests/i18n.test.tsx
new file mode 100644
index 0000000..87df3c5
--- /dev/null
+++ b/frontend/tests/i18n.test.tsx
@@ -0,0 +1,93 @@
+/**
+ * Task H4 – i18n-Grundlage: zentrale strings.ts Maps de/en.
+ *
+ * strings-Modul: t(key) mit de/en-Maps, getLanguage/setLanguage
+ * (localStorage-persistiert, Default de), subscribe + useT-Hook.
+ * Kern-UI-Texte (LayoutBar, Modus-Umschaltung) laufen ueber die
+ * Maps; vollstaendige mechanische Ersetzung aller Komponenten ist
+ * teilautomatisierbares Follow-up (Roadmap-Vermerk).
+ */
+import { describe, it, expect, vi, beforeEach, afterEach } from 'vitest';
+import { render, screen, cleanup, fireEvent, act } from '@testing-library/react';
+import {
+  t, getLanguage, setLanguage, subscribeLanguage, useT, LANGUAGES,
+} from '../src/i18n/strings';
+
+afterEach(cleanup);
+
+// ––– strings-Modul ––––––––––––––––––––––––
+
+describe('H4: i18n strings', () => {
+  beforeEach(() => {
+    localStorage.clear();
+    setLanguage('de');
+  });
+
+  it('Default-Sprache ist de', () => {
+    expect(getLanguage()).toBe('de');
+  });
+
+  it('t liefert deutsche Kern-UI-Texte', () => {
+    expect(t('layouts.title')).toBe('Layouts');
+    expect(t('layouts.add')).toBe('Layout hinzufügen');
+    expect(t('layouts.empty')).toBe('Keine Layouts');
+  });
+
+  it('setLanguage(en) wechselt auf englische Texte', () => {
+    setLanguage('en');
+    expect(t('layouts.title')).toBe('Layouts');
+    expect(t('layouts.add')).toBe('Add layout');
+    expect(t('layouts.empty')).toBe('No layouts');
+  });
+
+  it('Sprache persistiert in localStorage und ueberlebt Reload-Semantik', () => {
+    setLanguage('en');
+    expect(localStorage.getItem('webcad.language')).toBe('en');
+    // erneutes Lesen startet aus persistiertem Zustand
+    expect(getLanguage()).toBe('en');
+  });
+
+  it('LANGUAGES enthaelt genau de und en', () => {
+    expect(LANGUAGES).toEqual(['de', 'en']);
+  });
+
+  it('t mit unbekanntem Key liefert den Key zurueck (Fallback, lint-bar)', () => {
+    setLanguage('de');
+    expect(t('gibts.nicht')).toBe('gibts.nicht');
+  });
+
+  it('subscribeLanguage feuert bei Wechsel und unsubscribed', () => {
+    const cb = vi.fn();
+    const off = subscribeLanguage(cb);
+    setLanguage('en');
+    expect(cb).toHaveBeenCalledWith('en');
+    off();
+    cb.mockClear();
+    setLanguage('de');
+    expect(cb).not.toHaveBeenCalled();
+  });
+});
+
+// ––– useT-Hook + Komponente ––––––––––––––––––
+
+describe('H4: useT-Hook rendert LayoutBar zweisprachig', () => {
+  beforeEach(() => {
+    localStorage.clear();
+    setLanguage('de');
+  });
+
+  it('LayoutBar zeigt deutsche Texte und wechselt live auf englisch', async () => {
+    const LayoutBar = (await import('../src/components/LayoutBar')).default;
+    render(<LayoutBar doc={null} bridge={null} />);
+    expect(screen.getByText('Layouts')).toBeTruthy();
+    expect(screen.getByText('Keine Layouts')).toBeTruthy();
+    expect(screen.getByTitle('Layout hinzufügen')).toBeTruthy();
+
+    act(() => {
+      setLanguage('en');
+    });
+    // Live-Update via subscribe (nach act flush)
+    expect(screen.getByText('No layouts')).toBeTruthy();
+    expect(screen.getByTitle('Add layout')).toBeTruthy();
+  });
+});