Issue with next-intl locale switching for one colleague

[swajp]

Hi there,

I’m currently using next-intl in my Next.js project to handle language switching, and I’m encountering a peculiar issue that only affects one of my colleagues.
Configuration:
• defaultLocale is set to en
• localePrefix is set to as-needed

The problem:
When my colleague attempts to switch the language from en to cs, the /cs prefix gets removed from the URL, redirecting them back to /, and the language switches back to English (en). This issue doesn’t occur for me or any other team members.

For example:
• Expected behavior: Switching from /en to /cs should update the URL to include /cs and load the page in Czech.
• What happens for my colleague: The URL reverts to / (English default) instead of showing /cs.

We’ve checked basic things like clearing the browser cache and testing in incognito mode, but the issue persists specifically for their environment.

Also it works for them on my local server but on production or staging it doesn't.

[swajp]

I discovered that the issue is when the primary language is set to Czech and then user is not able to switch it, any idia?

[swajp]

export const routing = defineRouting({
    // A list of all locales that are supported
    locales: ["cs", "en"],

    // Used when no locale matches
    defaultLocale: "en",

    

    localePrefix: "as-needed"
})

[amannn]

Hmm, that's odd—can't see an issue with that. If /cs is present as a prefix, then this should be used.

Can you share a reproduction with a curl snippet that reproduces the behavior? (should contain the accept-language header as it should be observable in your colleagues network tools)

[swajp]

Okay it is not just a colleague problem, it is a global. When I set my browser's primary language to Czech the url is automatically example.com/cs and when I try to remove it, it is impossible it always revert it back to English (only on production)

https://github.com/swajp/next-intl-bug-repro-app-router

(test for production)
https://next-intl-bug-repro-app-router-rouge.vercel.app

[swajp]

when the browsers language is set to english there is no problem

[amannn]

Can you please share a curl snippet that helps me to reproduce this?

You can use "copy as curl" in the Chrome network tools:

I was unfortunately not able to reproduce: