Since to play an instrument in French is "Jouer de", I wonder where the de goes when it's interrogative. I think "Which instrument do you play" in French would be "Quel instrument jouez-vous" but there's no de. So, do you say "Quel instrument dont jouez vous" or "Dont instrument jouez vous". I just don't know where the de goes.
The grammatically correct formal sentence would be:
- De quel instrument jouez-vous ?
A still grammatical spoken French:
- De quel instrument est-ce que vous jouez ?
In non formal, real life, you'll more often hear the casual :
- Vous jouez de quel instrument ?
- Tu joues (de) quoi comme instrument ?