Hi Fabrym,
You could check the folder include.
First, review permission to folder and files to "666"
Then, look for file local_lang.php (in folder include)
it is supposed to be like this:
<?php
$languages = array(array('cz', 'Czech'), array('nl', 'Dutch'), array('en', 'English'), array('fi', 'Finnish'), array('fr', 'French'), array('de', 'German'), array('gr', 'Greek'), array('it', 'Italiano'), array('no', 'Norwegian'), array('pl', 'Polish'), array('sl', 'Slovenian'), array('es', 'Spanish'), array('sv', 'Swedish'));
$nb_langues = 13;
?>
If no, take a copy of that file in the Template .zip file.
(File could be corrupted when trying to use the manage page)
Regards,
Jacques