Users: Correct the value of the `lang` attribute in the admin area.

This corrects the value when the user's language is set to `English (United States)` but the site language is not.

Props ocean90, afercia

Fixes #42242

Built from https://develop.svn.wordpress.org/trunk@42220


git-svn-id: https://core.svn.wordpress.org/trunk@42049 1a063a9b-81f0-0310-95a4-ce76da25c4cd