To accept that we would need the word "nástroj"="instrument" in the Czech sentence.
No, we would not. On the contrary, the sentence "Na co vy/ty hrajete/hraješ" is first and most common translation of "What instrument do you play". The usage of "na co" actually implies that the sentence has neither of the following meanings: playing a game (board, card or computer), acting, cheating, gaming, betting or pretending (pretending is actually very similar, but requires a reflexive: "Na co si hraješ" = "Who are you pretending to be" both in a context of a child game and as pretending to be someone else).
Of course the MEANING is "which instrument do you play". No one has ever questioned that.
But we really, really cannot just accept any sentence with the same meaning. Really not. The translation must be as close as possible. And this one is simply too loose. See the Duolingo Golden rule.
Ok, so because I was downvoted over night, I must elaborate. I think your understanding of Golden Rule is confused, even by its simple definition from: https://forum.duolingo.com/comment/7117296
"What do you play?" is not a good translation of "Na co hrajete?" because there is a more natural translation "Which instrument do you play?", the "What do you play?" would be correct only in such case that it is already clear from the context that the talk is about musical instruments, but such a translation should be eliminated exactly becuase of the Golden Rule you cite.
This is, unless by explicit specification of the pronoun "vy" in the end, it is already signified that there was a previous debate over instruments but I don't think authors were being so subtle with the choice, or am I wrong?
A translation for which there exists a context where the translation best carries the meaning of the sentence in such context shoud be accepted. Simple as that. There is nothing loose on the translation "Which intrument do you play?" because it translates best the particular meaning. I am definitely reporting this translation.
You can play a game, a sport or you can play a musical instrument. When it is an instrument one plays ON it. Hraju NA piano. kytaru....
Hrát can also be reflexive (Hrát si na...) and that can mean two things. Hrajeme si na doktora = We are playing doctor. or Hrajeme si na piano = we are playing piano FOR ourselves (it is only us there, no audience).
Na co hrajete vy.... what (musical instrument) do you play?
I am a little bit confused. How to differentiate questions "Na co hrajete nejčastěji?" (na piano) and "Co hrajete nejčastěji?" (Bacha) in English?
I thought that it is:
What do you play on the most? What do you play the most? but the correct answer here for "Na co hrajete vy?" is without "ON".
Disclaimer: I am learning, too, so I may have something wrong here.
When "hrát" is used alone, it means to play anything EXCEPT a musical instrument. But when it's used with the preposition "na," it means specifically to play a musical instrument.
So I would translate BOTH "Na co hrajete nejčastěji?" and "Co hrajete nejčastěji?" as "What do you play most often." But in the first sentence, you'd be asking specifically about playing a musical instrument, while in the second you could be asking about a sport, a board game, cards, music, etc.
I definitely agree with the second part. However, even when "hrát" is used alone, it may still mean playing a musical intrument. Of course, it follows from the context, the sentence such as "Večer hraju." doesn't mean anything on its own, it may mean he is playing violin at the concert, or playing anything else really.
Neměla by se věta ''What do you play?'' překládat spíše jako ''Co hrajete?''
Toto je překlad z češtiny do angličtiny. V opačném směru je váš překlad možný.