Missing Language keys...how to detect

Code:
/**

 * Language Extension Class

 *

 * @package        CodeIgniter

 * @subpackage    Libraries

 * @category    Language

 * @author        Thomas Traub

 */

class MY_Language extends CI_Language {



    var $language    = array();

    var $is_loaded    = array();



    /**

     * Constructor

     *

     * @access    public

     */

    function MY_Language()

    {

        log_message('debug', "Language Extension Class Initialized");

    }



    // --------------------------------------------------------------------



    /**

     * Fetch a single line of text from the language array

     * 

     * Return the ident in case the string is not set

     *

     * @access    public

     * @param    string    $line     the language line

     * @return    string

     */

    function line($line = '')

    {

        $line = ($line == '' OR ! isset($this->language[$line])) ? $line : $this->language[$line];

        return $line;

    }



}