Unable to load language files

Once you retrieve the language from your session, you can set the default language to be used when you call $this->lang->load() by calling $this->config->set_item('language', $lang); (where $lang is the language to load). Then every call to $this->lang->load('whatever_lang_file'); afterwards in the same request will work as if you passed $lang as the second argument.

Bonfire

Practical CodeIgniter 3
CodeIgniter Testing Guide