"Kik" sounds a bit strange to me in this sentence, but I suppose it's just taken out of the context, isn't it? Let's say we're talking about an amount of people which we know and which are mentioned in a previous sentence, for example: "I don't know them very well. Who [of them] is walking among the old houses?". Is this how "kik" works here?
The Hungarian may make sense and be correct (I don't know as I'm not a native speaker- would appreciated insight from some here) but I don’t agree with your position on the English translation. When the interrogative pronouns 'who' and 'what' are used to ask for the subject of a clause, they should have singular verbs, even if the question expects a plural answer.
Q: Who is working tomorrow? A: John and Lucy (are working tomorrow)... Q: Who was at the party?... A: Emily and her friends A: What lives in those little holes...? A: Rabbits and spiders
Any native English speaker will tell you that asking “Who are working tomorrow?”, “Who were at the party” and “Who live in those little holes..?” sounds peculiar and uneducated.
However… When "who" and 'what' are used to ask for the complement of a clause, they can have plural verbs.
Q: Who are those tall men? A: Police officers Q: What are your happiest memories? A: Eating icecream at the beach and swimming in the ocean.
Of course in the right context (namely when using "who" as a conjunction) "who" and then a plural verb form can make sense e.g. Q"Who is walking among the old houses?" A " The women who live in the forest"
I appreciate your insight! I also apologize, I misconveyed my thoughts when I wrote that "who is vs. who are" is a common mistake in English. I'll attempt to better explain my reasoning and correct this false generality.
The English interrogative "who" is used with a singular verb because the word "who" itself lacks a distinction between plural and singular. We have one word to capture both, a dilemma not shared with Hungarian. For instance, the question in your first example can be properly answered with either one name or several, as "who" does not limit answers to one or the other. This adds a degree of ambiguity, which ambiguity promotes the general acceptance of a single verb conjugation for the sake of ease of use. Using a plural verb sounds incorrect because we English speakers accepted the singular conjugation as our solution and perpetuated its use, not because the plural is inherently incorrect.
Despite sounding awkward, using "who are" to ask for the subject of a clause is not incorrect for questions expecting plural responses (Michael Swan's "Practical English Usage" page 532). That said, I fully agree with you and the aforementioned text that "who is" sounds more natural for the reason I proposed earlier. That is absolutely the way I would ask this question myself. It's just not necessarily the way I would translate the Hungarian phrase.
I suggested the use of "are" rather than "is" since the "who" is specifically plural in the Hungarian phrase. This statement can't be answered with a single name and remain coherent conversationally, and requires a verb conjugated for plurality (the -nak conjugation of sétálni). Every relevant aspect of the response would also include this plurality, including the "who" (The women who live in the forest. - A nők, akik az erdőben laknak.). While awkward sounding, "who are" captures this plurality more precisely.
I imagine we're simply focusing on different aspects of translational accuracy, which is part of the fun of learning and comparing multiple languages.
I fully agree that "technically" there is no reason that "who are walking?" is incorrect but it would have to be considered either an archaic or rare usage which is why I wouldn't encourage it as an English translation of this sentence.. however you make an excellent point- if "kik sétálnak" makes grammatical sense in Hungarian, then the statement presumes an antecedent i.e. the person uttering the sentence has a priori knowledge of several people walking among the old houses. By contrast "ki sétál" implies either a singular or unknown subject walking among the old houses. I think the only English translation that comes close to capturing the true sentiment of the sentence but without the awkwardness of "Who are walking..." then would be "Who are they, walking among the old houses?
As you point out though, it depends on what aspects of translation accuracy are important to you, and in this case I think it is somewhat subjective.