deps/v8/src/js/i18n.js
function lookupMatcher(service, requestedLocales) {
  if (IS_NULL(InternalRegExpMatch(GetServiceRE(), service))) {
    throw MakeError(kWrongServiceType, service);
  }

  // Cache these, they don't ever change per service.
  if (IS_UNDEFINED(AVAILABLE_LOCALES[service])) {
    AVAILABLE_LOCALES[service] = getAvailableLocalesOf(service);
  }

  for (var i = 0; i < requestedLocales.length; ++i) {
    // Remove all extensions.
    var locale = InternalRegExpReplace(
        GetAnyExtensionRE(), requestedLocales[i], '');
    do {
      if (!IS_UNDEFINED(AVAILABLE_LOCALES[service][locale])) {
        // Return the resolved locale and extension.
        var extensionMatch = InternalRegExpMatch(
            GetUnicodeExtensionRE(), requestedLocales[i]);
        var extension = IS_NULL(extensionMatch) ? '' : extensionMatch[0];
        return {'locale': locale, 'extension': extension, 'position': i};
      }
      // Truncate locale if possible.
      var pos = %_Call(StringLastIndexOf, locale, '-');
      if (pos === -1) {
        break;
      }
      locale = %_Call(StringSubstring, locale, 0, pos);
    } while (true);
  }

  // Didn't find a match, return default.
  return {'locale': GetDefaultICULocaleJS(), 'extension': '', 'position': -1};
}
function lookupMatcher(service, requestedLocales) {
  if (IS_NULL(InternalRegExpMatch(GetServiceRE(), service))) {
    throw MakeError(kWrongServiceType, service);
  }

  // Cache these, they don't ever change per service.
  if (IS_UNDEFINED(AVAILABLE_LOCALES[service])) {
    AVAILABLE_LOCALES[service] = getAvailableLocalesOf(service);
  }

  for (var i = 0; i < requestedLocales.length; ++i) {
    // Remove all extensions.
    var locale = InternalRegExpReplace(
        GetAnyExtensionRE(), requestedLocales[i], '');
    do {
      if (!IS_UNDEFINED(AVAILABLE_LOCALES[service][locale])) {
        // Return the resolved locale and extension.
        var extensionMatch = InternalRegExpMatch(
            GetUnicodeExtensionRE(), requestedLocales[i]);
        var extension = IS_NULL(extensionMatch) ? '' : extensionMatch[0];
        return {'locale': locale, 'extension': extension, 'position': i};
      }
      // Truncate locale if possible.
      var pos = %_Call(StringLastIndexOf, locale, '-');
      if (pos === -1) {
        break;
      }
      locale = %_Call(StringSubstring, locale, 0, pos);
    } while (true);
  }

  // Didn't find a match, return default.
  return {'locale': GetDefaultICULocaleJS(), 'extension': '', 'position': -1};
}
Returns best matched supported locale and extension info using basic
lookup algorithm.