code: function getOptimalLanguageTag(original, resolved) {
    // Returns Array<Object>, where each object has maximized and base properties.
    // Maximized: zh -> zh-Hans-CN
    // Base: zh-CN-u-ca-gregory -> zh-CN
    // Take care of grandfathered or simple cases.
    if (original === resolved) {
      return original;
    }
  
    var locales = %GetLanguageTagVariants([original, resolved]);
    if (locales[0].maximized !== locales[1].maximized) {
      return resolved;
    }
  
    // Preserve extensions of resolved locale, but swap base tags with original.
    var resolvedBase = new GlobalRegExp('^' + locales[1].base, 'g');
    return InternalRegExpReplace(resolvedBase, resolved, locales[0].base);
  }
docs: It's sometimes desireable to leave user requested locale instead of ICU
  supported one (zh-TW is equivalent to zh-Hant-TW, so we should keep shorter
  one, if that was what user requested).
  This function returns user specified tag if its maximized form matches ICU
  resolved locale. If not we return ICU result.