What is the best order to check locales in setlocale?

Question

PHP

Right now I'm using the following code to set the locale:

$locale = 'en_US';
if (!setlocale(LC_ALL, $locale.'.UTF-8', $locale.'.utf-8', $locale.'.UTF8', $locale.'.utf8', $locale)) {
  $lang = preg_replace('!_.*!', '', $locale);
  if (!setlocale(LC_ALL, $lang.'.UTF-8', $lang.'.utf-8', $lang.'.UTF8', $lang.'.utf8', $locale)) {
    // Set current locale
    setlocale(LC_ALL, '');
  }
}

I would like to speed up the process and arrange the locales by popularity.