I think we are talking about his ethnicity here. It is worth noting that terms like 'nationality' are differently understood today in Poland and in Western Europe. 'Nationality' in the West is what we see as 'citizenship' in Poland. Our 'nationality', in turn, is what they see as 'ethnicity'. So the question is: was Copernicus ethically German as there is no doubt that he was, felt and he acted as a true citizen of Poland.
First, there was no such thing as 'Germany' in his time. Obviously, there was the Holy Roman Empire, but that wasn't exactly the same as 'Germany'. Second, the Teutonic Order was a unit of international nature with, yes, the majority of knights being of German origin. Third, the concept of a national state would look strange in the times of Copernicus (neither Poland nor Germany nor the Teutonic Order were national states at that time) as this concept is only a 19th century invention. You people seem to apply modern concepts to former times whereas the people of those former times did not see things and did not define themselves through these modern terms.
We can only and safely say that Copernicus was born 'Polish citizen' and bore allegiance to the Polish Crown which in modern terms would mean he had a Polish citizenship and held a Polish passport :-).
At the same time he never studied at a German university -just Polish and Italian
That's irrelevant. Latin was the language of universites at that time rather than German, Polish or Italian. Universities were in towns, so to speak, rather than in states.
Copernicus also belonged to the German fraternity when he studied in Italy
For me, this is something talking in favor for his German ethnicity. Do you know if he had left behind any texts (letters, for example) written in German beside his well-known works in Latin?