Optional language parameter in a ZF2 route

gistfile1.php

<?php
// route configuration, for instance in a module.config.php
// ...
             'home' => array(
                'type' => 'segment',
                'options' => array(
                    'route' => '/[:lang/]',
                    'defaults' => array(
                        '__NAMESPACE__' => 'Application\Controller',
                        'controller'  => 'Index',
                        'action'      => 'index',
                        'lang'        => 'en',
                    ),
                    'constraints' => array(
                        'lang' => 'en|fr'
                    ),
                ),
                'may_terminate' => true,
                'child_routes' => array(
                    'page-one' => array(
                        'type' => 'segment',
                        'options' => array(
                            'route' => 'page-one',
                            'defaults' => array(
                        		'controller' => 'Application\Controller\Index',
                        		'action'     => 'page-one',
                    		),
                        ),
                    ),
					'page-two' => array(
                        'type' => 'segment',
                        'options' => array(
                            'route' => 'page-two',
                            'defaults' => array(
                        		'controller' => 'Application\Controller\Index',
                        		'action'     => 'page-two',
                    		),
                        ),
                    ),
                ),
            ),
// ...