Date popup will lose translate of it after a certain time

http://www.eurotrekking.hu/en/online-registration

If you take look at the "When" field, this is a date popup, that is good when you are on english page. But when you change the language(http://www.eurotrekking.hu/hu/online-jelentkezes) the popup wont be translated.

I have already updated the date module many times but this doesnt help. If i cleared the cache everything would be fine. But after some time the date popup will forget that it has been already translated.

Drupal: 7.21

If i can provide any more info just let me know. Thx.