"Who won the championship?"

Translation: Kto wygrał mistrzostwa?

[FredFukada]

Is mistrzostwa genitive? I'd rather have expected accusative here...

[alik1989]

It's accusative plural.

https://en.wiktionary.org/wiki/mistrzostwo#Declension

https://forum.duolingo.com/comment/14878229?comment_id=44653766

[EdwardMich543202]

Why does the English source use the singular, if the target meaning is plural?

[Jellei]

The Polish word is grammatically plural, but the meaning is still singular, one tournament. Which I believe is singular "championship" in English.