This is a demo repository of vercel/next.js#53724 issue with a minimal example.

Reproduction

npm install
npm run dev

Test Pages Router:

1. Visit http://localhost:3000/, works as expected
2. Visit http://localhost:3000/pl, works as expected

Test App Router:

1. Visit http://localhost:3000/en/about, returns 404, should return the app/[lang]/about/page.jsx page
2. Visit http://localhost:3000/pl/about, returns 404, should return the app/[lang]/about/page.jsx page
3. Visit http://localhost:3000/asdf/about, returns the app/[lang]/about/page.jsx page (asdf is not a defined locale in next.config.js)

Now, remove the i18n key from next.config.js. The App Router will start working, however the Pages Router will stop working.

Expected behaviour

It should be possible to adopt App Router incrementally alongside Pages Router for multi-language pages. The mere existence of i18n configuration in next.config.js shouldn't affect the App Router.