code: function canonicalizeLanguageTag(localeID) {
    // null is typeof 'object' so we have to do extra check.
    if ((!IS_STRING(localeID) && !IS_RECEIVER(localeID)) ||
        IS_NULL(localeID)) {
      throw MakeTypeError(kLanguageID);
    }
  
    // Optimize for the most common case; a language code alone in
    // the canonical form/lowercase (e.g. "en", "fil").
    if (IS_STRING(localeID) &&
        !IS_NULL(InternalRegExpMatch(/^[a-z]{2,3}$/, localeID))) {
      return localeID;
    }
  
    var localeString = TO_STRING(localeID);
  
    if (isValidLanguageTag(localeString) === false) {
      throw MakeRangeError(kInvalidLanguageTag, localeString);
    }
  
    var tag = %CanonicalizeLanguageTag(localeString);
    if (tag === 'invalid-tag') {
      throw MakeRangeError(kInvalidLanguageTag, localeString);
    }
  
    return tag;
  }
docs: Canonicalizes the language tag, or throws in case the tag is invalid.