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
* ...
*/
?>