this will require some hand coding and is not a supported feature. you will need to have an understanding of coding PHP to get this to work.
you would need to store the selected langauge in a session variable and create a copy of the
CSSMenuWriter/cssmw_content/menu.php
File, in the copy of the menu file, edit the meu items to the second language.
Then in the template, create an if statement to determine which menu to use:
<?php if(isset($_SESSION['language']) && $_SESSION['language'] == 'es') { ?>
<div id="topNavigation"><?php require_once("../CSSMenuWriter/cssmw_content/menues.php"); ?></div>
<?php } else { ?>
<div id="topNavigation"><?php require_once("../CSSMenuWriter/cssmw_content/menu.php"); ?></div>
<?php } ?>