Using the universal “they” is okay in a sentence such as “How do they say hello in Croatia?” However, this sentence does not refer to a specific group of people, it’s more like “How do people describe this?” Since “they” does not refer to a specific group of people, Duolingo seems to consider it incorrect.
Sometimes I forget that using the plural means "how does one..." or "how do you..." - and so I wrote "How are they describing it?" and was marked wrong. Is it? I can imagine saying this to a friend: "How are they (say, the police) describing it (a situation where a person was accidentally killed)?"
איך מתארים את זה?