[firelight]

Ok, I think I found it.
I changed the admin/category_modify.php by adding the following:

define('USE_TRUSTED_SCRIPT_VARS',1);

to have this as result:

define('USE_TRUSTED_POST_VARIABLES',1);
$trusted_post_variables = array("category_lng_description","category_new_des cription","description");
define('USE_TRUSTED_SCRIPT_VARS',1);

This definately allowed me to insert javascript in the category description.

Any input is still very welcome because I'm not a programmer.

Thanks.