deps/v8/src/js/i18n.js
function lookupSupportedLocalesOf(requestedLocales, availableLocales) {
  var matchedLocales = new InternalArray();
  for (var i = 0; i < requestedLocales.length; ++i) {
    // Remove -u- extension.
    var locale = InternalRegExpReplace(
        GetUnicodeExtensionRE(), requestedLocales[i], '');
    do {
      if (!IS_UNDEFINED(availableLocales[locale])) {
        // Push requested locale not the resolved one.
        %_Call(ArrayPush, matchedLocales, requestedLocales[i]);
        break;
      }
      // Truncate locale if possible, if not break.
      var pos = %_Call(StringLastIndexOf, locale, '-');
      if (pos === -1) {
        break;
      }
      locale = %_Call(StringSubstring, locale, 0, pos);
    } while (true);
  }

  return matchedLocales;
}
function lookupSupportedLocalesOf(requestedLocales, availableLocales) {
  var matchedLocales = new InternalArray();
  for (var i = 0; i < requestedLocales.length; ++i) {
    // Remove -u- extension.
    var locale = InternalRegExpReplace(
        GetUnicodeExtensionRE(), requestedLocales[i], '');
    do {
      if (!IS_UNDEFINED(availableLocales[locale])) {
        // Push requested locale not the resolved one.
        %_Call(ArrayPush, matchedLocales, requestedLocales[i]);
        break;
      }
      // Truncate locale if possible, if not break.
      var pos = %_Call(StringLastIndexOf, locale, '-');
      if (pos === -1) {
        break;
      }
      locale = %_Call(StringSubstring, locale, 0, pos);
    } while (true);
  }

  return matchedLocales;
}
Returns the subset of the provided BCP 47 language priority list for which
this service has a matching locale when using the BCP 47 Lookup algorithm.
Locales appear in the same order in the returned list as in the input list.