我可以在手机上的菜单列表后添加PHP字符串吗？

I'm setting up a custom language switcher with WPML and I have a problem to insert that string:

wpml_custom_language_selector();

after the list of menu items, out of UL tag

I tried, but without success, to insert the string on functions.php

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

function add_search_to_nav()
{  
     $string = wpml_custom_language_selector(); 
return $string;
}