Chouby on "[Theme: Twenty Eleven] Problems in translation with plugin Polylang"

Unfortunately, the Catalan version is not updated on the official languages repository (http://svn.automattic.com/wordpress-i18n/ca/tags/). So Polylang can not download the language file automatically of your theme. The language file is probably available from somewhere else but you will have to upload it manually. Check here to start:
http://ca.wordpress.org/