Uplift #42349, #42771: Use web exposed locales instead of regional locales where appropriate

Uplift Patchset

Gitlab Issue(s)

• #42349 (closed)
• #42771 (closed)

Upstream Mozilla Issue(s):

• https://bugzilla.mozilla.org/show_bug.cgi?id=1746668

Notes

MR: !1080 (merged)

Our patch has been accepted by Mozilla, except that another person found possible problems with it after it was accepted: https://bugzilla.mozilla.org/show_bug.cgi?id=1746668#c18.

So, I did not ask to land it, and we should write an improved patch for it.