That would be really great if someone could explain me why my variables are ignored when inserted in $Config['UserFilesPath'] such as :
$Config['UserFilesPath'] = "/" . $GLOBALS['current_site'] . "/" . $GLOBALS['ext_lang'] . "/dir_userfiles/";
A directory called "dir_userfiles" is created in the site root directory which shows that the first two variables are ignored. The same things happen with session variables.
Variables ignored in "upload" config file