code: function isValidLanguageTag(locale) {
    // Check if it's well-formed, including grandfadered tags.
    if (IS_NULL(InternalRegExpMatch(GetLanguageTagRE(), locale))) {
      return false;
    }
  
    // Just return if it's a x- form. It's all private.
    if (%_Call(StringIndexOf, locale, 'x-') === 0) {
      return true;
    }
  
    // Check if there are any duplicate variants or singletons (extensions).
  
    // Remove private use section.
    locale = %_Call(StringSplit, locale, '-x-')[0];
  
    // Skip language since it can match variant regex, so we start from 1.
    // We are matching i-klingon here, but that's ok, since i-klingon-klingon
    // is not valid and would fail LANGUAGE_TAG_RE test.
    var variants = new InternalArray();
    var extensions = new InternalArray();
    var parts = %_Call(StringSplit, locale, '-');
    for (var i = 1; i < parts.length; i++) {
      var value = parts[i];
      if (!IS_NULL(InternalRegExpMatch(GetLanguageVariantRE(), value)) &&
          extensions.length === 0) {
        if (%_Call(ArrayIndexOf, variants, value) === -1) {
          %_Call(ArrayPush, variants, value);
        } else {
          return false;
        }
      }
  
      if (!IS_NULL(InternalRegExpMatch(GetLanguageSingletonRE(), value))) {
        if (%_Call(ArrayIndexOf, extensions, value) === -1) {
          %_Call(ArrayPush, extensions, value);
        } else {
          return false;
        }
      }
    }
  
    return true;
   }
docs: Validates the language tag. Section 2.2.9 of the bcp47 spec
  defines a valid tag.
  
  ICU is too permissible and lets invalid tags, like
  hant-cmn-cn, through.
  
  Returns false if the language tag is invalid.