You have a mistake in the translation - "Who work at the markets?" is not correct English.
According to Michael Swan "Practical English usage, Oxford 1995": when 'who' and 'what' are used to ask for the subject of a clause they most often have singular verbs, even if the question expects a plural answer. So, there there seems to be nothing inherently 'ungrammatical' with using a plural verb with an interrogative 'who'. I'd say that using "who" for "whom" is a horrible mistake, however colloquial that might be.
Who are working in the markets should be accepted. The subject and verb are both plural, so your translation should be also.
I think ''Who are working'' is odd sounding, but technically correct if you know it is multiple people.
Mizinamo is correct. "Who are working in the markets" is not correct English. If you spoke this to someone you'd get a funny look.
"in the markets" would we acceptable in English and not chnge the meaning