"Do you promise to speak Dutch?"
Translation:Beloven jullie om Nederlands te spreken?
Look at the conjugation of the verb: beloven julllie, beloof jij/je, belooft u (formal you). How did you get to level 25 in Dutch without knowing this?
It's meant probabaly the other way round: How do you know if the english "you" is supposed to be translated either as jij or jullie? The answer is: You don't. But usually Duolingo accepts both options.