Translation doesn't work!

My WordPress (3.9) is in german and I also confirmed that the wp-config.php has:
define('WPLANG', 'de_DE');
as described in the plugin doc under:
http://support.time.ly/how-can-i-change-edit-or-translate-the-language/

Still the plugin shows only in english… (all of it is in english)

https://ww.wp.xz.cn/plugins/all-in-one-event-calendar/