Isn't this literally "His girlfriend is a Polish woman (or female)"?
It is. However, English often says just He is English, He is Czech, He is Polish, He is French...
OK, that is true, but as it is not unknown to express it as "... is a Polish woman (maybe also "girl" as another option) " should that version not be accepted by Duolingo?
a Polish woman is polská žena...
It is also accepted here.
His girlfriend is Pole. What's wrong? Pole is one of the translations of the word Polka.