Next.js 16 App router internationalization root error

After following the guide on: https://nextjs.org/docs/app/guides/internationalization The translation logic itself seems to work, but the redirection from the root will never happen.

When visiting localhost:3000 (localhost:3000/) it will always end up giving back a 404 error. Visiting localhost:3000/en correctly gives back the english version of the root page, and visiting localhost:3000/de correctly gives back the german version.

If i understand this correctly the problem must be caused by the proxy that is not redirecting correctly but after spending a lot of time on changing things and even trying to explicitly handle pathname === "/" I still cant make it work, changing the matcher to multiple different suggestions also didnt work.

Regarding the project structure, everything is moved under app/[lang]/ as per guide. So the root page also lives under app/[lang]/page.tsx as well as root layout app/[lang]/layout.tsx

I am stuck with this and would appreciate any help and a possible explanation of as why the root is not redirected to the default locale language. (e.g localhost:3000 => should redirect to localhost:3000/de in my case since de is specified as default)

This is my current proxy.ts (which is almost identical to the guide):

import { NextResponse } from "next/server"; import { match } from "@formatjs/intl-localematcher"; import Negotiator from "negotiator"; const locales = ["de", "en"]; const defaultLocale = "de"; function getLocale(request: Request) { const negotiatorHeaders: Record<string, string> = {}; request.headers.forEach((value, key) => (negotiatorHeaders[key] = value)); const languages = new Negotiator({ headers: negotiatorHeaders }).languages(); return match(languages, locales, defaultLocale); } export function proxy(request: any) { const { pathname } = request.nextUrl; if ( locales.some( (loc) => pathname === `/${loc}` || pathname.startsWith(`/${loc}/`), ) ) { return; } const locale = getLocale(request); request.nextUrl.pathname = `/${locale}${pathname}`; return NextResponse.redirect(request.nextUrl); } export const config = { matcher: [ // Skip all internal paths (_next) "/((?!_next).*)", // Optional: only run on root (/) URL // '/' ], };