function new_nav_menu_items($items, $args) { 
    $lang = get_bloginfo("language");

    if( $args->theme_location == 'top-navigation' ){
	// en-US
	$lang_link = '<li class="menu-item"><a href="/zh/">中文</a></li>'; 

	if ( $lang == 'zh-CN' ){ 
	   $lang_link = '<li class="menu-item"><a href="/en/">English</a></li>';
	} 

        $items = $items.$lang_link;
    }

    return $items;
}

add_filter( 'wp_nav_menu_items', 'new_nav_menu_items', 10, 2 );