An idea for allowing learners to collaborate on long sentences via partial translations.
If a sentence is long and you don't feel like translating the entire thing and/or you get stuck on part(s) of it, then maybe you could translate just some of it and click "Submit partial translation".
You could also ask Duolingo to show you a list of partial translations before you submit anything. You could then pick one and edit it and submit it either as a partial translation or a complete translation.
Any time someone contributes to a partial translation that you worked on, you would be notified showing you the partial translation before and after the modification. You could then accept the changes and submit it as a partial translation or a complete translation.
Your translation would only be graded once you submit it as a complete translation.