How to retrieve $config['myConfig_array'] in Code Igniter

I've followed Codeigniter language and all seems to be setup as a hook.

function pick_language() {

    require_once(APPPATH.'/config/language.php');

    session_start();

    // Lang set in URL via ?lang=something
    if(!empty($_GET['lang']))
    {
        // Turn en-gb into en
        $lang = substr($_GET['lang'], 0, 2);
        $_SESSION['lang_code'] = $lang;
    }

    // Lang has already been set and is stored in a session
    elseif( !empty($_SESSION['lang_code']) )
    {
        $lang = $_SESSION['lang_code'];
    }

    // Lang has is picked by a user.
    // Set it to a session variable so we are only checking one place most of the time
    elseif( !empty($_COOKIE['lang_code']) )
    {
        $lang = $_SESSION['lang_code'] = $_COOKIE['lang_code'];
    }

    // Still no Lang. Lets try some browser detection then
    else if (!empty( $_SERVER['HTTP_ACCEPT_LANGUAGE'] ))
    {
        // explode languages into array
        $accept_langs = explode(',', $_SERVER['HTTP_ACCEPT_LANGUAGE']);

        log_message('debug', 'Checking browser languages: '.implode(', ', $accept_langs));

        // Check them all, until we find a match
        foreach ($accept_langs as $lang)
        {
            // Turn en-gb into en
            $lang = substr($lang, 0, 2);

            // Check its in the array. If so, break the loop, we have one!
            if(in_array($lang, array_keys($config['supported_languages'])))
            {
                break;
            }
        }
    }

    // If no language has been worked out - or it is not supported - use the default
    if(empty($lang) or !in_array($lang, array_keys($config['supported_languages'])))
    {
        $lang = $config['default_language'];
    }

    // Whatever we decided the lang was, save it for next time to avoid working it out again
    $_SESSION['lang_code'] = $lang;

    // Load CI config class
    $CI_config =& load_class('Config');

    // Set the language config. Selects the folder name from its key of 'en'
    $CI_config->set_item('language', $config['supported_languages'][$lang]['folder']);

    // Sets a constant to use throughout ALL of CI.
    define('CURRENT_LANGUAGE', $lang);
}

Now when I try to access $config['supported_languages'] it returns null or errors. Why?