"Who is in the department stores?"
Translation:Kik vannak az áruházakban?
Youre right the translation "who is" (singular) is "ki van" (singular), not "kik vannak" (multiple).
I would never use "who is" if im refering to multiple people.
Instead i would restructure the sentence to say "who are the peoppe in the department store".
I think this is another question that needs to get revisited.
why would someone use plural in this sentence?
If you suspect that the answer will be plural?
Most department stores will have more than one person inside at any given time...
In some varieties of English, there's "who all" which is a plural of "who", used in questions such as "Who all was at the party?", where you expect more than one person as the answer.