cache & session

[eluser]szazman[/eluser]
hi
i have language name in session eg.
$newdata = array(
'username' => 'johndoe',
'email' => '[email protected]',
'logged_in' => TRUE
'lang' => 'en'
);
$this->session->set_userdata($newdata);

I would like to use cache but when i use caching i can change the language only after cache expires.
How to make caching all languages and read only the one that exists in session???