Custom post_fields replaced by default language

Hi,
custom post_fields are overwritten by default language when it has been edited second time, ex:

1. Set meta box text in default language. Save post.
2. Switch to another language and set translation. Save post.

.. and everything works till:

3. Switch to the default language. Change default text or just save post.
4. Switch to another language and previously translated text has been replaced with the one set in step 3.

Example code:

// in wpm-config.json:
"post_fields": {
  "page_meta_description": {}
}

// meta box value:
echo get_post_meta($post_id, 'page_meta_description', true);

// meta box save:
update_post_meta($post_id, 'page_meta_description', sanitize_text_field($_POST['page_meta_description']));