"Are they thirsty?"
Translation:Onko heillä jano?
That means "Are they thirst?". Note that "jano" is a noun, not an adjective. You could say "Ovatko he janoisia?", which has the same meaning and it's perfectly understandable, but it's an unusual way of phrasing it. When it comes to certain states of being, an ownership clause is typically used to express that someone has it. The usual structure of an ownership clause is adverbial-verb-subject, where the adverbial is in adessive case and expresses the owner (in this case "heillä"), the subject is in nominative or partitive case and expresses the owned thing (in this case "jano"), and the verb is "olla" conjugated according to the subject as usual so it therefore has to be in third person. The verb assumes first position in this instance because it's a question. This literally translates to "do they have thirst?", just like the German equivalent "Haben sie Durst?".