frontend/src/app/app.config.ts

@@ -29,6 +29,8 @@ import { serverOriginInterceptor } from './core/interceptors/server-origin.inter
 import { catchError, firstValueFrom, of } from 'rxjs';
 import { StaticTranslateLoader } from './core/i18n/static-translate.loader';
 import {
+  getNavigatorLanguagePreferences,
+  parseAcceptLanguage,
   resolveInitialLanguage,
   SUPPORTED_LANGS,
 } from './core/i18n/language-resolution';
@@ -70,6 +72,11 @@ export const appConfig: ApplicationConfig = {
         (typeof request?.url === 'string' && request.url) || router.url || '/';
       const lang = resolveInitialLanguage({
         url: requestedUrl,
+        preferredLanguages: request
+          ? parseAcceptLanguage(readRequestHeader(request, 'accept-language'))
+          : getNavigatorLanguagePreferences(
+              typeof navigator === 'undefined' ? null : navigator,
+            ),
       });
 
       return firstValueFrom(
@@ -88,3 +95,21 @@ export const appConfig: ApplicationConfig = {
     provideClientHydration(withEventReplay()),
   ],
 };
+
+function readRequestHeader(
+  request: {
+    headers?: Record<string, string | string[] | undefined>;
+  } | null,
+  headerName: string,
+): string | null {
+  if (!request?.headers) {
+    return null;
+  }
+
+  const headerValue = request.headers[headerName.toLowerCase()];
+  if (Array.isArray(headerValue)) {
+    return headerValue[0] ?? null;
+  }
+
+  return typeof headerValue === 'string' ? headerValue : null;
+}

frontend/src/app/core/services/language.service.spec.ts

@@ -7,6 +7,7 @@ import {
 } from '@angular/router';
 import { TranslateService } from '@ngx-translate/core';
 import { LanguageService } from './language.service';
+import { RequestLike } from '../../../core/request-origin';
 
 describe('LanguageService', () => {
   function createTranslateMock() {
@@ -82,9 +83,14 @@ describe('LanguageService', () => {
     const translate = createTranslateMock();
     const router = createRouterMock('/calculator?session=abc');
     const navigateSpy = router.navigateByUrl as unknown as jasmine.Spy;
+    const request: RequestLike = {
+      headers: {
+        'accept-language': 'it-CH,it;q=0.9,en;q=0.8',
+      },
+    };
 
     // eslint-disable-next-line @typescript-eslint/no-unused-vars
-    const service = new LanguageService(translate, router);
+    const service = new LanguageService(translate, router, request);
 
     expect(translate.use).toHaveBeenCalledWith('it');
     expect((translate as any).setFallbackLang).toHaveBeenCalledWith('it');
@@ -97,31 +103,41 @@ describe('LanguageService', () => {
     expect(navOptions.replaceUrl).toBeTrue();
   });
 
-  it('uses the default language on the root URL', () => {
+  it('uses the preferred browser language on the root URL', () => {
     const translate = createTranslateMock();
     const router = createRouterMock('/');
     const navigateSpy = router.navigateByUrl as unknown as jasmine.Spy;
+    const request: RequestLike = {
+      headers: {
+        'accept-language': 'de-CH,de;q=0.9,en;q=0.8,it;q=0.7',
+      },
+    };
 
     // eslint-disable-next-line @typescript-eslint/no-unused-vars
-    const service = new LanguageService(translate, router);
+    const service = new LanguageService(translate, router, request);
 
-    expect(translate.use).toHaveBeenCalledWith('it');
+    expect(translate.use).toHaveBeenCalledWith('de');
     expect(navigateSpy).toHaveBeenCalledTimes(1);
 
     const firstCall = navigateSpy.calls.mostRecent();
     const tree = firstCall.args[0] as UrlTree;
-    expect(router.serializeUrl(tree)).toBe('/it');
+    expect(router.serializeUrl(tree)).toBe('/de');
   });
 
   it('uses the default language for non-root URLs without a language prefix', () => {
     const translate = createTranslateMock();
     const router = createRouterMock('/calculator?session=abc');
     const navigateSpy = router.navigateByUrl as unknown as jasmine.Spy;
+    const request: RequestLike = {
+      headers: {
+        'accept-language': 'de-CH,de;q=0.9,en;q=0.8,it;q=0.7',
+      },
+    };
 
     // eslint-disable-next-line @typescript-eslint/no-unused-vars
-    const service = new LanguageService(translate, router);
+    const service = new LanguageService(translate, router, request);
 
-    expect(translate.use).toHaveBeenCalledWith('it');
+    expect(translate.use).toHaveBeenCalledWith('de');
     expect(navigateSpy).toHaveBeenCalledTimes(1);
 
     const firstCall = navigateSpy.calls.mostRecent();

frontend/src/app/core/services/language.service.ts

@@ -1,4 +1,4 @@
-import { Injectable, signal } from '@angular/core';
+import { Inject, Injectable, Optional, REQUEST, signal } from '@angular/core';
 import { TranslateService } from '@ngx-translate/core';
 import {
   NavigationEnd,
@@ -6,7 +6,12 @@ import {
   Router,
   UrlTree,
 } from '@angular/router';
-import { resolveInitialLanguage } from '../i18n/language-resolution';
+import {
+  getNavigatorLanguagePreferences,
+  parseAcceptLanguage,
+  resolveInitialLanguage,
+} from '../i18n/language-resolution';
+import { RequestLike } from '../../../core/request-origin';
 
 type SupportedLang = 'it' | 'en' | 'de' | 'fr';
 type LocalizedRouteOverrides = Partial<Record<SupportedLang, string>>;
@@ -23,6 +28,7 @@ export class LanguageService {
   constructor(
     private translate: TranslateService,
     private router: Router,
+    @Optional() @Inject(REQUEST) private request: RequestLike | null = null,
   ) {
     this.translate.addLangs(this.supportedLangs);
     this.translate.setFallbackLang('it');
@@ -37,6 +43,11 @@ export class LanguageService {
     const initialTree = this.router.parseUrl(this.router.url);
     const initialLang = resolveInitialLanguage({
       url: this.router.url,
+      preferredLanguages: this.request
+        ? parseAcceptLanguage(this.readRequestHeader('accept-language'))
+        : getNavigatorLanguagePreferences(
+            typeof navigator === 'undefined' ? null : navigator,
+          ),
     });
     this.applyLanguage(initialLang);
     this.ensureLanguageInPath(initialTree);
@@ -137,7 +148,7 @@ export class LanguageService {
       const queryLang = this.getQueryLang(urlTree);
       const rootLang = this.isSupportedLang(queryLang)
         ? queryLang
-        : this.defaultLang;
+        : this.currentLang();
       if (rootLang !== this.currentLang()) {
         this.applyLanguage(rootLang);
       }
@@ -169,6 +180,17 @@ export class LanguageService {
     return typeof lang === 'string' ? lang.toLowerCase() : null;
   }
 
+  private readRequestHeader(headerName: string): string | null {
+    const headerValue =
+      this.request?.headers?.[headerName.toLowerCase()] ??
+      this.request?.get?.(headerName.toLowerCase());
+    if (Array.isArray(headerValue)) {
+      return headerValue[0] ?? null;
+    }
+
+    return typeof headerValue === 'string' ? headerValue : null;
+  }
+
   private isSupportedLang(
     lang: string | null | undefined,
   ): lang is SupportedLang {

frontend/src/server-routing.spec.ts

@@ -1,8 +1,8 @@
 import { resolvePublicRedirectTarget } from './server-routing';
 
 describe('server routing redirects', () => {
-  it('redirects the root path to the default language', () => {
-    expect(resolvePublicRedirectTarget('/')).toBe('/it');
+  it('does not handle the root path because it is resolved separately', () => {
+    expect(resolvePublicRedirectTarget('/')).toBeNull();
   });
 
   it('redirects unprefixed public pages to the default language', () => {

frontend/src/server-routing.ts

@@ -13,7 +13,7 @@ export function resolvePublicRedirectTarget(pathname: string): string | null {
     normalizedPath === '/' ? '/' : normalizedPath.replace(/\/+$/, '');
   const segments = splitSegments(trimmedPath);
   if (segments.length === 0) {
-    return `/${DEFAULT_LANG}`;
+    return null;
   }
 
   const firstSegment = segments[0].toLowerCase();

frontend/src/server.ts

@@ -5,6 +5,10 @@ import { dirname, join, resolve } from 'node:path';
 import { fileURLToPath } from 'node:url';
 import bootstrap from './main.server';
 import { resolveRequestOrigin } from './core/request-origin';
+import {
+  parseAcceptLanguage,
+  resolveInitialLanguage,
+} from './app/core/i18n/language-resolution';
 import { resolvePublicRedirectTarget } from './server-routing';
 
 const serverDistFolder = dirname(fileURLToPath(import.meta.url));
@@ -37,6 +41,25 @@ app.get(
   }),
 );
 
+app.get('/', (req, res) => {
+  const userAgent = req.get('user-agent');
+  const preferredLanguages = parseAcceptLanguage(req.get('accept-language'));
+  const lang = resolveInitialLanguage({
+    preferredLanguages,
+  });
+  const stableRedirect = shouldUseStableRootRedirect(
+    userAgent,
+    preferredLanguages,
+  );
+
+  res.setHeader('Vary', 'Accept-Language, User-Agent');
+  res.setHeader('Cache-Control', 'private, no-store');
+  res.redirect(
+    stableRedirect ? 308 : 302,
+    `/${stableRedirect ? 'it' : lang}${querySuffix(req.originalUrl)}`,
+  );
+});
+
 app.get('**', (req, res, next) => {
   const targetPath = resolvePublicRedirectTarget(req.path);
   if (!targetPath) {
@@ -83,3 +106,21 @@ function querySuffix(url: string): string {
   const queryIndex = String(url ?? '').indexOf('?');
   return queryIndex >= 0 ? String(url).slice(queryIndex) : '';
 }
+
+function shouldUseStableRootRedirect(
+  userAgent: string | undefined,
+  preferredLanguages: readonly string[],
+): boolean {
+  return preferredLanguages.length === 0 || isLikelyCrawler(userAgent);
+}
+
+function isLikelyCrawler(userAgent: string | undefined): boolean {
+  const normalized = String(userAgent ?? '').toLowerCase();
+  if (!normalized) {
+    return false;
+  }
+
+  return /(bot|crawler|spider|slurp|bingpreview|google-read-aloud)/.test(
+    normalized,
+  );
+}