"It is just a matter of time."
Translation:To tylko kwestia czasu.
Question can be pytanie or kwestia but matter can't be translated as pytanie. Besides, it is an idiom and there isn't really any different way to express it (just like the English phrase).
You're right. I didn't think about it. In Polish, however, only this one idiom is widely used. But still, although question can mean pytanie (inquiry) or kwestia (matter), they're not interchangeable.
In your example, "question" doesn't mean "question" in the sense of "pytanie", but is synonymous with "matter", and would also translate as "kwestia".
'To jest tylko sprawa czasu' is suggested as a correct answer, but 'to tylko sprawa czasu' is marked incorrect - why is that?