So, in Hungarian you never use the singular for these types of questions? E.g. "Ki dolgoz a bankban?"
Ok great. But which one do you use normally, the singular or the plural?
I think this questions is a very artificial one, so it is rarely appear in the everyday situations. But if you know that only one people is working on the bank and you want to know his/her name, you can use the singular form. If you know that multiple people are working in the bank, you use the plural form ( e.g. you are a police inspector and from the list of suspects you want to have the list of people who are working in the bank. )
While "who work" or "who are working" is unusual in English, I could easily come up with situations where it would be acceptable, e.g., in response to a list of people that the listener doesn't recognize. "X and Y and Z work in the bank." "Excuse me, WHO work in the bank?" It's enough of a grey area that a course to teach English speakers Hungarian shouldn't put itself in quite so much of a position of grading the English of its users but should concentrate on getting the Hungarian part consistent.