As someone who sometimes does the proofreading of legal texts (I'm going to translate them myself some day) I thought I'd share a few thoughts with you :)
> I hereby authorize tax advisor XY
"Adviser; advisor. The -er spelling is sanctioned over the -or spelling in the dictionaries(...)" [Brian Garner, Dictionary of modern legal usage].
> to a person registered for tax in Poland and to represent me in the
> related proceedings.
In legal texts, I will usually come across the full name of a country, e.g. the United Kingdom instead of England or, in this case, the Republic of Poland instead of Poland. Also, I'm not entirely sure but I think that, depending on the context, "a person registered for tax purposes" is more specific.
And one additional question: Is it possible to entitle the document "power of attorney" if there is no attorney? In the above-mentioned case it is a tax advisor and his client. Or does the word "attorney" mean "the person who is authorized" here and not a legal representative?
Yes, if the document is entitled as above, then there has to be an attorney, but not necessarily a lawyer. Attorney (sometimes called attorney in fact) is simply "one with power of attorney to act for another; legal agent." Attorney-at-law, however, means a person is a licensed lawyer.
I hope this helps :)
Student or Learner