On the Zend Developer Zone there's a new post talking about including language information in your Zend Framework application's default route in a cleaner manner.
There are several ways how to include language id in default route of Zend Framework. However, generally you end up with the solution not quite elegant and likely not totally trouble-free. I have seen people overwriting the default route by new one which mimics module route with additional language id. There is no need to throw the default module route away to do this. To get it right chain the plain language route with default route.
He gives code examples of the routing code to put in your bootstrap that uses the Zend_Controller_Router_Route_Chain and a plugin to handle the language checking and routing handling.