From https://gitlab.torproject.org/tpo/applications/tor-browser/-/issues/40280#note_2723329. [MozIntl](https://bugzilla.mozilla.org/show_bug.cgi?id=1635561) switched from `AppLocale` to `RegionalPrefLocales` in `xpc_LocalizeRuntime` (https://hg.mozilla.org/integration/autoland/rev/05b8c9d0b50d). This is obtained from `https://hg.mozilla.org/integration/autoland/rev/05b8c9d0b50d`. In that function, when `intl.regional_prefs.use_os_locales = false` (the default), the os regional locale will still be used if the language subtag matches the app locale (e.g. en-CA and en-US). We may want to change this and only use OS locale if `intl.regional_prefs.use_os_locales = true`.