"Es un hombre sentado en una silla."
In many dialects (mainly speaking for British English here) we say 'is sat' to describe the present state as the ongoing result of a past action. 'was sat' implies it was true in the past, but isn't now.
I think it only works for a few verbs (sat and stood are the only ones that come to mind right now), but it's similar to how we say 'the car is parked'. If you think about it it makes more sense - why would you say 'the lamp is standing in the corner'? It's not continually doing anything, the act of standing and then standing some more, someone stood it there and it continues to be in that state.
Bit of a language tangent there, but yeah - 'is sat' is incredibly common English in some parts of the world, so it should be accepted I reckon.
Yes, you are right, it is used dialectically in some parts of England, and it would be understood by native English speakers (with perhaps a wince from the pedants). But it is not standard English and not correct usage. Duolinguo are doing pretty well accepting standard British and US usage, if they start accepting the millions of various dialects it would be a never-ending task.
Well it's regularly used in TV and print media (including on the BBC) so it's not some kind of regional anomaly or anything - it's a part of normal, everyday British English.
We could get into a philosophical discussion about what exactly constitutes 'standard English' and whether anyone speaks it (they don't), but at the end of the day Duolingo's here to teach us Spanish or whatever, and recognise natural English input from the user. If someone uses a common, widely used and recognised phrasing to express the correct answer, then it shouldn't be rejected.
I can understand if it's not added automatically as a valid answer, but it should be accepted when people report it at least.