So, maybe I understood this wrong?
Jesus, I just read it and my head hurts.
From what you're saying, it sounds as if you do indeed have to withhold 20% if the country doesn't have a taxation treaty with Poland. Germany should do, so you wouldn't have to withhold the money. As for copywriting - this is tangible, so it isn't under the scope of this.
An accountant is probably the best bet here, it may very well be an obscure tax that no-one bothers to collect.