I do not see how reader can determine third person is a he or a she. Please explain.
you can't. In real life you know from context. In this excercise you use he/she/it whatever you want.
I guess duolingo has no way of knowing that you mean a singular 'they' instead of just making a grammatical mistake...
Just to inform the expression "Idziemy do łóżka", or similar, has a very sexual meaning in Polish language :). A better expression would be "Idziemy spac" A very credible source (my polish wife) has told me that.
I don't know if "very". Sure, it can easily mean that, but it can easily be innocent as well. But okay, let's say that "Idziemy spać" may be safer.
The audio sounds fine to me, plus "łyżka" would be not only quite illogical, but also wrong grammatically here.
Isn't the pronoun necessary here, since the conjugation isn't unique to any pronoun?
It is not necessary, but less redundant than other pronouns.
This sentence is OK if from previous sentence, or context you can tell who is the subject.
Of course in duolingo we have no context, we have to imagine some.
No, although of course it would be useful. It would probably be known from the context, although there's also a possibility that the sentence is gonna remain ambiguous.
It's practically the same in Russian, but it's awkward to just say Idjot w krowat' without on/ona/ono or context, that's why I asked.
The pronoun would make it clear here whether it is "he" or "she" going to bed.
Since you are not specifying if on/ona/ono is going to bed, can it be translated to "is going to bed"?
No. The Polish sentence is a complete sentence with the pronoun omitted. In English, that's not a complete sentence.
It's one of the starred answers. Either you had a typo somewhere that you hadn't noticed, or it was a bug.
Is it also a way to say it in Polish as well as English? (In my language, we would never say 'going to bed', we just say 'about to sleep'.)