`Language pack 'en_US' not installed` in the dev tools console

[Jeremy Tuloup]

Description

JupyterLab often shows the following warning in the dev tools console:

Reproduce

Using 4.2.0rc0 on Binder.

But this is also reproducible from the latest commit on main.

Expected behavior

There should likely be no warning in the dev tools.

Context

• Operating System and version: Ubuntu 23.10
• Browser and version: Latest Firefox and Chrome
• JupyterLab version: 4.2.0rc0

[Jeremy Tuloup]

This happens even if the Accept-Language header includes en-US:

This also seems to be the case with 3.6.3, so it's been there for quite a while:

[Jeremy Tuloup]

The message originates from:

jupyterlab/packages/translation/src/manager.ts

Line 54 in 59acb33

 console.warn(message); 

[Jeremy Tuloup]

Looks like this logic was last modified in #13721

cc @fcollonval @krassowski