Autoload language codeigniter
$config['language']
is the default folder used for loading language files, which is why your
fr_lang.php
is loaded from there.
Either change the value of:
$config['language']
when needed, like:
$this->config->set_item('language', 'value');
(Remember this has to be done before the languages load, so you would use a hook for that http://codeigniter.com/user_guide/general/hooks.html.)
Or else, load your language files on the fly:
$this->lang->load('filename', 'language');
You should definitely check out the core classes to get a better understanding of how things work. You can browse the code easily here: https://github.com/EllisLab/CodeIgniterFor instance, the languages are loaded with this class: https://github.com/EllisLab/CodeIgniter/blob/develop/system/core/Lang.php
If you need any more help, let me know.