Translation:The Italian man is selling wine to the Frenchman.
"French" can be used as a noun to denote the people of France or the French language (see http://www.thefreedictionary.com/french), but I'm not sure whether it can be used in the meaning "a (single!) person from France, Frenchman"... Hence I would translate "Włoch sprzedaje wino Francuzowi" = "The Italian sells wine to the Frenchman" (one French person) and "The Italian sells wine to the French" = "Włoch sprzedaje wino Francuzom" (many French people).
I agree that it should be Frenchman instead of French but also I don't think it should be "The Italian" either! I would never say "The Italian is selling wine..." It seems faintly rude to me. Like being Italian is the only important thing about this person. I would be somewhat annoyed to be referred to as "the British".
It's like you wouldn't say "The bald is selling wine" or "The deaf is selling wine"
I would say "The Italian man is selling wine" if I wanted to point it out to someone (for some reason haha).
Sounds like either this is some hidden camera documentary like Big Brother or like they're in a nature documentary. Or part of some comedy film where a German an Italian and an Australian go on a road trip and their nationality is their whole character (probably in a massive caricature).
Maybe it's just my version of English! :)