Avoid using regional OS locales

From #40280 (comment 2723329).

MozIntl 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.