How to set the difficulty automaticaly (On Litecoin and assimilate) ?

Just find the $config['difficulty'] line on the global.inc.php file and replace by that :

$config['difficulty'] = shell_exec ('litecoind -datadir=/home/{your_username}/.litecoin/ getdifficulty');

EDIT: Please note, with variable difficulty in MPOS for a while, this isn't really needed anymore. If you database table is storing the share difficulty, that value is used for all calculations and the set config difficulty is ignored.