[all-in-one-event-calendar] Translation / language problem

Hello, again thanks for the great work.
Despite the incomplete documentation I followed the instructions to the fullest I downloaded the required .po and .mo files uploaded them to the language folder of the plugin (there were only the defaults).
Renamed them all-in-one-event-calendar-xx_XX.po
all-in-one-event-calendar-xx_XX.mo.

edite wp-config.php with the following:

define('WPLANG', 'xx_XX');

But nothing happened. Am I missing something or please help me find the cause of this issue.

THanks