Unit test for js locale

Let's introduce an automated regression test to ensure that the javascript.use_us_english_locale pref is applied to

members of window.Date objects
Intl API
toLocaleString
DateTimeFormat.formatToParts

I can work on this at some point. The problem is that we need to set the LANG variable to de_DE or another non-en_US locale before starting Firefox. Maybe boklm's testing framework can do this? Or maybe it's possible in Mozilla's automated testing system, but I don't know a way yet.

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