Append custom WPML switcher to primary menu.

wpml_appendto_primary_menu.php

<?php

add_filter('wp_nav_menu_items', 'itc_wp_nav_menu_items_filter', 10, 2);
 
function itc_wp_nav_menu_items_filter($items, $args){
    if($args->theme_location == 'primary'){
        $items .= '<li class="menu-item menu-item-language menu-item-language-current">';
        $items .= itc_icl_post_languages();
        $items .= '</li>';
        return $items;
    }else{
        return '';
    }
}
function itc_icl_post_languages(){
    $languages = icl_get_languages('skip_missing=0&orderby=code');
    if(1 < count($languages)){
        $langs = array();
        foreach($languages as $l){
            if($l['language_code'] == 'zh-hans') $l['language_code'] = "CN";
            $active = $l['active'] == 1?"class='active_language'":"";
            $langs[] = '<a '.$active.' href="'.$l['url'].'">'.$l['language_code'].'</a>';
        }
        return  '<span class="languages">' . join("/", $langs) .'</span>';
    }
}