Accept-Language (i18n) header not evaluated when directly accessing sub-routes

[mleimer]

What version of Next.js are you using?

10.0.5

What version of Node.js are you using?

14.15.1

What browser are you using?

Chrome
Version 88.0.4324.104 (Official Build) (64-bit)

What operating system are you using?

Windows

How are you deploying your application?

next build & next start

Describe the Bug

Accept-Language headers of browser are not (correctly) evaluated when directly accessing a sub-page through a deep-link or alike.

Expected Behavior

Accept-Language headers of browser should be evaluated upon first access to the Next.JS application independently of the entry point (root/index or any other sub page)

Given

• Request-Header containing Accept-Language: en,de-CH;q=0.9,de-DE;q=0.8,de;q=0.7,en-US;q=0.6
• next.config.js being

module.exports = () => {
  return {
    i18n: {
      locales: ['de', 'en'],
      defaultLocale: 'de',
    },
  };
};

When

• directly accessing http://localhost:3000/posts/ssg-ssr

Then

• should set locale within useRouter() as 'en'

To Reproduce

Use example from tutorial:
npx create-next-app nextjs-blog --use-npm --example "https://github.com/vercel/next-learn-starter/tree/master/basics-final"

Remove external data-fetching calls.

Add next.config.js with the following configuration:

module.exports = () => {
  return {
    i18n: {
      locales: ['de', 'en'],
      defaultLocale: 'de',
    },
  };
};

Print locale on different pages using

const {locale} = useRouter();
console.log(locale);

---

1.) Deploy above application
2.) Open http://localhost:3000/posts/ssg-ssr (or any other sub-page)
3.) locale within useRouter() should be 'en', but is 'de'

Probably a fourth case of #20488

[mleimer]

I just identified that this is probably rather a feature request, as the docs limit said behavior to the root page.

When a user visits the application root (generally /), Next.js will try to automatically detect which locale the user prefers based on the Accept-Language header and the current domain.

[romaincointepas]

I've opened a feature request at #22361.

[balazsorban44]

This issue has been automatically locked due to no recent activity. If you are running into a similar issue, please create a new issue with the steps to reproduce. Thank you.