If you call this method with an empty string, it will return a list of all locales available in the ICU library (via the intl extension):
<?php
print_r(ResourceBundle::getLocales(''));
 /* Output might show
  * Array
  * (
  *    [0] => af
  *    [1] => af_NA
  *    [2] => af_ZA
  *    [3] => am
  *    [4] => am_ET
  *    [5] => ar
  *    [6] => ar_AE
  *    [7] => ar_BH
  *    [8] => ar_DZ
  *    [9] => ar_EG
  *    [10] => ar_IQ
  *  ...
  */
?>