Translation:Who is this person?
Not exactly: the former means "who is this?" and the latter means "who is this person?" because the latter indicates "人". In some contexts, e.g., when the question is clearly asking about a person, yes, those questions are the same, but, if a child is introducing you to a pet cat or a puppet for example, the former question would be more appropriate than the latter.
If the DuoLingo app does not accept the former, maybe it's because one of the points of this lesson is to understand how 个 and 人 function.
In English, a demonstrative determiner agrees in number with its referent, that is, with the word it introduces. The plural of "this" is "these," and the plural of "that" is "those:"
- I like that song. ("that" is a singular demonstrative determiner)
- I like those songs. ("those" is a plural demonstrative determiner)
or, in the case of this question, the relevant verb also agrees in number with the referent, but the interrogative pronoun ("who") remains the same in both singular and plural:
- Who is this person? (singular)
- Who are these people? (plural)
In Chinese, 这个人是谁? means "Who is this person?" To specify "Who are these people?" you could change 个 to 些, but keep 人, 是, and 谁 unchanged:
This pattern differs from English, in which "is" would have to become "are," "person" would have to become "people," and there is no "numerary adjunct" (个 or 些) at all, but in a way, "this," 这个, becomes "these," 这些. I believe you could also append the suffix 们 to emphasize the plurality of 人: 这些人们是谁? but I am not sure that adding 们 is necessary.
In modern English, the plural form of "person" is "people," rather than "persons," except in certain technical or legal applications. Without any context, as in the case of this exercise question, my natural, overwhelming inclination would be to ask "Who are these people?" rather than "Who are these persons?" but again, I think the best translation of this exercise question is the singular, "Who is this person?"
In modern English, there is sometimes a subtle or technical distinction between "persons" and "people" (generally, we say "people" rather than "persons" unless we are specifically drawing the distinction or using "persons" in its technical, e.g., legal, context).
For instance, "person" is sometimes used in referring to someone's body or to the items the person is presently carrying, often used with "on:" "The suspect had no drugs on his person," meaning, the person was not in possession of any drugs. So, in the plural, I suppose you could say, "The people had no drugs on their persons," but notice that the first plural is "people" rather than "persons." Generally, I would avoid using "persons" unless I knew that the technical (e.g., legal or medical) sense of the word was called for. Certainly, you should always use "people" rather than "persons" to refer to people generally, or as organizations, e.g., in all of these examples, "people" is correct, and "persons" would be incorrect:
- The people have spoken.
- The People's Republic of Cork.
- The people will elect a new President in 2020.
- Why can't people just relax and be cool?
I do not know how or even whether Chinese makes this same distinction; so, I am sorry I cannot help with specifying "persons" as opposed to the general "people" in Chinese.
Historically, it was commonly held that "persons" was the plural of "person", while "people" was a generic plural. So, you could say "all people are welcome" or "we invited six persons".
Usage has changed in the past century, and while legal convention is conservative in its use of these words, most style guides today prefer the word "people" over "persons" always.
In short, "persons" is correct, but quickly becoming archaic usage.
There are a couple of cases perhaps worth mentioning where these words really aren't interchangeable:
As a collective noun, always use "people" (e.g., " the Chinese people").
When using "person" to refer to a body (e.g., "he had it on his person"), "persons" is the preferred plural.
It's misleading to suggest that persons is correct. As you note this is an archaic usage which is still occasionally heard only in specific legal contexts. To use persons outside of that context is definitely not standard English and most speakers will think of it is an error. Learners of English should not use persons (unless they're using correctly it in a specific legal context).
For anyone having trouble understanding "This" and "That" in chinese I recommend going on YouTube and search up Learn Mandarin: The basic Mandarin toolkit (Fluenz Channel). She does a great job in explaining the two "This" and "That". She also demonstrate on how to say it in a sentence.
There's too much ambiguity with "guy", because in some English, it only refers to men. 人 indicates a person, but not a gender. 男人 would be "man", and 女人 would be "woman". There's a lot of slang for dude/guy like 老铁, 哥们儿, 兄弟, but it's best to learn things properly before using slang.