We accept both 'shall' and 'will', but we have to add them individually and so haven't got around to adding 'shall' in all instances. Please submit reports whenever you come across them, it is the only way we can change them. The sentence discussions are not the place to report alternative translations :)
Thanks for your reply, Joanne. Have dug a bit more and found a can of worms! E.g. a lengthy piece on Wikipedia headed 'shall and will'. Have only recently discovered Duolingo and hope that I'm posting this in the right place. Must add that I'm finding the lessons incredibly user-friendly (borderline addictive!), and am much heartened by the attention being paid to minority languages. Many thanks to all of you in the team.