Avoid using intl.locale.requested pref directly
We wrongly assume that
intl.locale.requested contains just a single locale, but it's actually a list of locales if more than one language pack is installed (which we are allowing for now).
We should probably use JS API
Services.locale.requestedLocale and the appropriate C++ API (need to check) instead of using
intl.locale.requested value. This affects "torbutton" code and patch for #14631 (moved).