Changing language on the fly

[eluser]PoWah[/eluser]
I have autoloaded one language file in autoload.php:

Code:

$autoload['language'] = array('application');

There are in this file common constants that I need in my application permanently.
How I can change language on the fly? Tried

Code:

$this->config->set_item('language', 'nl');

but it didn't work :/ CI still outputs default language lines