Automatic Language dectection seems to work only in Chrome when locale is only partially in locales

[alveshelio]

Bug report

In Firefox or Safari, automatic language detection doesn't seem to work properly if in locales we have ["pt-PT"] but Accept-Language is "pt" for example

According to the documentation: https://nextjs.org/docs/advanced-features/i18n-routing#automatic-locale-detection

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.

However, if in next.config.js we have locales: ['en-CA', 'fr-CA', 'pt-PT'] and we've defined the browser language to pt then NextJS will fail to set locale as 'pt-PT' and will instead set locale as the defaultLocale

So unless the browser language is exactly the same as in locales, automatic language detection won't work.

Clone this example https://github.com/vercel/next.js/tree/canary/examples/i18n-routing

Then change locales to something like ["en-CA", "fr-CA", "pt-PT"] or a language of your choice but it needs to be xx-XX and set `defaultLocale: "en-CA"

Then change your browser language to one of the languages in locales but instead make sure you only have xx, for example en or fr or pt or whatever languages you've added to your locales.

Navigate to the index page and you should be redirected to http://localhost:3000/pt or the language you've set your browser to.

Instead you stay on http://localhost:3000

I'm not sure it's a bug in NextJS though since it works in Chrome but fails in both Firefox and Safari.

[theostrahlen]

I have the same issue but also in Chrome.
I have locales: ['en-US', 'sv-SE'] in my next.config.js but automatic detection for Swedish only works if I change it to sv.

[ijjk]

Closing in favor of #18676

[balazsorban44]

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