you can retrieve this identifier with that code :
$defaultLanguage = pll_default_language();
$translations = pll_get_post_translations($post_ID);
$id = $translations[$defaultLanguage];
you can retrieve this identifier with that code :
$defaultLanguage = pll_default_language();
$translations = pll_get_post_translations($post_ID);
$id = $translations[$defaultLanguage];