Oh yes, I know this and as I said it works well also with a .po/.mo file without any translations plugin.
I just wanted to know if it was possible to make the translation work without deleting the field definitions from the builder (as happens for custom taxonomy labels) but from your answer I understand that it is not possible. Thanks anyway.
I solved using wp_get_attachment_image() for the English translation only and for two field only. It's a very strange thing because for all other custom image advacend everything works well. Thanks.
