switching default lang in config.php

[eluser]Ngulo[/eluser]
hi all,

i'm wondering how to switch language in my site...

I've some different language packs in my /language directory.

this is the scheme:

Code:

-language
    -en
    ----en_lang.php
    -pt
    ----pt_lang.php
    -sp
    ----sp_lang.php

now i would like to switch the default language into config.php ($config['language'])
by a session data that i use to set like this:

Code:

$param = $this->uri->segment(3);
$this->session->set_userdata('site_lang',$param)

so where i can retrieve session->userdata('site_lang'); and put my new session site language for all helpers ,libraries and so on... ??
thanks guys