There is no `general.useragent.locale` anymore
general.useragent.locale
and intl.locale.matchOS
are gone. We need to use the new intl.locale.requested
pref instead. See: https://bugzilla.mozilla.org/show_bug.cgi?id=1414390 and https://groups.google.com/forum/#!topic/mozilla.dev.platform/5Udhquypu14.