I found this useful Internationalization code:
everything works well except I am unable to use CI functions inside this class .
I want to set $languages and $special variable from DB .
but when I am using $CI =& get_instance(); in instance function its showing following error :
Fatal error: Class 'CI_Controller' not found in /system/core/CodeIgniter.php on line 231
The language class is loaded before the CodeIgniter instance exists, which is why you get the error.
You can use a post_controller_constructor hook to set your variables.
Here is a thread from the CodeIgniter forums where someone is tried to do something similar: http://codeigniter.com/forums/viewthread/108639/