<?php
function duplicate_und_translate($post_id, $lang)
{
    $exists = pll_get_post($post_id, $lang);
    if (!$exists) {

        $title = get_the_title($post_id);
        $oldpost = get_post($post_id);

        $post = array(
            'post_title' => $title,
            'post_status' => 'publish',
            'post_type' => $oldpost->post_type,
            'post_content' => $oldpost->post_content,
            'post_author' => 1
        );
        $new_post_id = wp_insert_post($post);


        $data = get_post_custom($post_id);
        foreach ($data as $key => $values) {
            foreach ($values as $value) {
                add_post_meta($new_post_id, $key, $value);
            }
        }

        PLL()->model->set_post_language($new_post_id, $lang);
        PLL()->model->post->save_translations($new_post_id, array('en' => $post_id));

        wp_update_post([
            'ID' => $new_post_id,
            'post_name' => $oldpost->post_name
        ]);

    }
}