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

[jtpio]

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

[jtpio]

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:

[jtpio]

The message originates from:

jupyterlab/packages/translation/src/manager.ts

Line 54 in 59acb33

console.warn(message);

[jtpio]

Looks like this logic was last modified in #13721

cc @fcollonval @krassowski