Review Mozilla 1846224: Make Javascript locale spoofing fine-grained and configurable per-realm

Bugzilla: https://bugzilla.mozilla.org/show_bug.cgi?id=1846224

This is the bug where javascript.use_us_english_locale was removed. It caused various regressions for which we already have issues open (e.g., #42349 (closed)).

We might either solve them first, or mark this (or them) as duplicates.