I would agree that the autonomous form is most often, but not necessarily always, the best translation of the English sentence; therefore, it should be accepted (if it is not already).
Of course, the deciding factor between this and the equally grammatically correct literal translation is context. Duolingo lacks this, and that is part of its charm; thus, both should be accepted.
The situation in reverse is further complicated by the lack of an agent in Irish, making "One speaks Russian in Moscow" the more literal but less idiomatic translation of the autonomous form in English. A passive translation is also possible but still less idiomatic that the active English translation here, especially for those of an Orwellian outlook.
The active translation in English wins out as the most idiomatic, yet the equally active but agent-free sentence wins in Irish. This is a qualified conclusion, however, assuming that the autonomous form in Irish is the most likely in absence of any context to support or refute that. There are different schools of thought on translation, particularly in the context of L2 acquisition, to muddy the waters further.
On reflection, I suspect that not all of the above was considered and implemented when this sentence was composed originally and translated. If it had, I doubt the course would be released yet. We are still working on it, but I hope you all can appreciate that it takes an infinite amount of time to perfect everything.
The internet is not the best means of communication, but I feel that the Team is not getting the benefit of the doubt, both on and off this site. Morale is vital to the success of any project but especially one that depends on volunteers. If you do support our aims in providing a course free of charge, and I am speaking generally here, please do work with us to improve it but please do so with this in mind.
If you have understood everything here, then you probably are not in need of this course: maith thú! If not, I have included a link at the bottom for further reference, but please be advised that it's a dry read and your time as a learner is better invested watching some TV. That might seem counterintuitive, but if your aim is to speak Irish, this will bring higher returns per unit time invested at this stage: some grammar is taught, but more is caught, for better or worse. If you need proof, try listening to anyone.
I believe that I’ve understood your points, but perhaps that’s only because I find such dry reads to be both interesting and essential to aiding my comprehension, since I wasn’t raised in an Irish-speaking environment and have had no formal education to learn Irish. I’ve noted elsewhere that my primary goal here is to improve my understanding of the written language rather than the spoken language, so that influences my feedback in error reports and in the discussion areas. I think that most people here would like this course to be as good as it can be, so please don’t feel that it’s a case of not giving the benefit of the doubt; my guess is that in most cases, when people notice something that they consider to be suboptimal, they’d like to see it improved to make the course that much better. Those people who have a strong foundation in Irish (would that I were one) are also volunteers, doing what they can to help make the course get closer to emerging from beta.
Of course the translations of context-free sentences can be inexact, but since in this instance the question was an Irish to English translation, was the expected English translation the starting point for composing the Irish sentence, or was it the other way around? Those of us who aren’t involved in course creation don’t know the process by which the content is determined.
The autonomous verb is used in several other sentences in this lesson expressing similar concepts, e.g. 'Itear rís i Moscó', 'Úsáidtear an Euro sa Spáinn'. Why this sentence was translated so literally from the English 'They speak Russian in Moscow' I don't know.
If the intended meaning was something like Scilling's suggestion above then rephrasing it slightly would have been much clearer: 'Labhraíonn siad Rúisis nuair a bhíonn siad i Moscó', perhaps.