To retrieve an item from your config file, use the following function:
$this->config->item('item name');
Where item name is the $config array index you want to retrieve. For example, to fetch your language choice you'll do this:
$lang = $this->config->item('language');
The function returns FALSE (boolean) if the item you are trying to fetch does not exist.
If you are using the second parameter of the $this->config->load function in order to assign your config items to a specific index you can retrieve it by specifying the index name in the second parameter of the $this->config->item() function. Example:
// Loads a config file named blog_settings.php and assigns it to an index named "blog_settings"
$this->config->load('blog_settings', TRUE);
// Retrieve a config item named site_name contained within the blog_settings array
$site_name = $this->config->item('site_name', 'blog_settings');
// An alternate way to specify the same item:
$blog_config = $this->config->item('blog_settings');
$site_name = $blog_config['site_name'];