Translation:The population in this country is young.
Duo does not accept, "The population of this country is young," which is a more natural expression in English.
I see your point but it doesn't flow in English as a standalone sentence. Maybe in context as part of contrast between the population in this country vs that country but even still that's stretching it. It could also work if it was a population of a specific group in the country but that's again dependent on contextual information not present here. "the population of this country" should be accepted.
A standalone sentence?? Almost all of these example sentence we encounter make more sense as snippets from a conversation. You just gave the perfect example of why one might say 'the population IN this country'. So as long as you can provide a possible context for sentence, then it is valid.
Dont worry, English learners. You can say, IN this country, and everyone will undestand you perfectly.
While it would certainly be understood to say, "the population in this country," I agree that it's not a natural sentence. The only case where I can see it making sense is if a single population were spread over multiple countries, so you were specifically referring to the population in THIS country—unless the French phrase is also used in that very particular way, "of this country" is a more natural translation.
I put "the population in this country ARE young" which i feel should be accepted. In British English it is common to are over is when referring to a collective noun, "The team are playing well" "This generation are more tech savy". I accept that "is" is more common in US english and would expect both to be accepted?
Nobody ever says "the population in this country". The preposition is always "of". Don't mark us wrong when you are the one who are wrong.
I agree, the population of is the equal but more common use in US English and should be accepted. Duo often demands even longer stretches of translation with more dramatic French variation from English.
The probability of two (or more) words occurring together is called "collocation" This is mainly what determines whether "population of" or "population in" sounds more natural.
That's about all I know, but there might be other learners here who know how to analyze the degrees of collocation. That would be a way to end these pointless he said/she said arguments about what "sounds more natural".