Immersion: How to get learners to care about the quality of their translations.
An obvious reason to care is so that you would get helpful feedback via email from subsequent edits by others.
However, what if many of the edits make the translation worse? That would not be helpful feedback.
And so it is important to try to ensure that each time a translation is edited, the person doing the editing is more skilled than all people who have worked on that sentence translation thus far.
Duolingo can use progress in the lessons as a measure of how skillful the learner is likely to be.
Moreover, Duolingo could use a more accurate measure by also testing people on their (claimed) native language.