error in interpolation when language is set to "en_US"

[yardz]

Description

I'm trying to use next-intl but I run into a bug. My language is set to the "en_US" format. When we use the language in this format and try to do an interpolation, this error occurs: Error: INVALID_MESSAGE: Incorrect locale information provided

I reproduced the error in this repository:
https://github.com/yardz/example-app-router-without-i18n-routing--error

It only has 3 commits.
The first is a copy of the example example-app-router-without-i18n-routing (the base example: https://github.com/amannn/next-intl/tree/main/examples/example-app-router-without-i18n-routing).
The second commit is changing the name of the language files (even the second commit is working normally)
In the third commit I just add the variable for interpolation, then I start getting the error

Verifications

• I've verified that the problem I'm experiencing isn't covered in the docs.
• I've searched for similar, existing issues on GitHub and Stack Overflow.
• I've compared my app to a working example to look for differences.

Mandatory reproduction URL

https://github.com/yardz/example-app-router-without-i18n-routing--error

Reproduction description

The quickest way to reproduce is:

Change the language files from "en" to "en_US" and "de" to "de_DE".
And make the necessary adjustments (this commit: yardz/example-app-router-without-i18n-routing--error@7d7c70a) ---- At this stage the example you provide is working normally ----

Then add a variable for interpolation (This commit: yardz/example-app-router-without-i18n-routing--error@b700d49)

Use this example as starting point: https://github.com/amannn/next-intl/tree/main/examples/example-app-router-without-i18n-routing

Expected behaviour

the interpolation works with file "en" but does not work with the pattern "en_US". I believe it should also work with the "en_US" pattern

[amannn]

Locales should use - to separate a language and a region (e.g. en-US).

In v4, next-intl will warn early when an invalid locale is encountered.

[yardz]

@amannn I believe that's not why. All translations work with "_" (underscore) only interpolations do not work. If that were the case, all translations should stop working, right?

I put a print. You can see that "Login" is working but the interpolation is not.

[amannn]

In v3 the behavior is a bit inconsistent. Some features might still work which don't rely on the Intl API, but others will break.

The error you're seeing comes from the JavaScript runtime itself:

In v4 I'll make it more obvious if you have an incorrect locale, so you don't run into this error down the road.

[vadxq]

But en-US or zh-CN doesn't work either, and the error message is: EXPECT_SELECT_ARGUMENT_SELECTOR_FRAGMENT ({locale, select, en {English} zh-CN {Chinese} other {Unknown}})

Nextjs15 + react19, with this example: https://github.com/amannn/next-intl/tree/main/examples/example-app-router

[vadxq]

Now, I use a temporary solution, hoping that the v4 version will support

You can use the following solution for adaptation:

selecter
{t('locale', { locale: cur.replace('-', '') })}

en.json
"locale": "{locale, select, en {English} zhCN {中文} other {Unknown}}"

[amannn]

I see your point, and I'd wish ICU select arguments would support dashes, but there are unfortunately two standards here that next-intl relies on:

1. BCP 47 locales don't allow underscores (as mentioned above)
2. ICU select arguments don't allow dashes (ref)

I agree it's not ideal and I'm also using the .replace workaround, but it's the situation we have to work with currently.