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).

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