From 2015 to 2022, I spent hundreds of hours on Duolingo, translating articles, answering language questions on the forums, and helping to improve the smaller courses by reporting mistakes.
There are thousands of volunteers who donated their labour to Duo: the course creators who wrote their courses, the volunteers who created grammar guides (some smaller languages had an entire second course in the forums), the wiki contributors, the native speakers who answered questions in the sentence discussions.
All of their work made Duolingo the powerhouse it is today. Duo was built by a community who believed in its original mission: language learning should be free and accessible.
Bit by bit all of our work was hidden from us as Duolingo became a publicly-traded company. And now that work is being fed into their AI as training data.
Well, I've learned the true lesson of Duolingo: never give a corporation your labour for free. Don't ever trust them, no matter what they say. Eventually greed will consume any good intentions.
I would suggest that what you really learned was to ALWAYS "License" your contributions to any crowd-sourced project under CC-BY-SA, the #FSF#GPL or #AGPL, or some other #Creative_Commons or strong #Copyleft license.
Again: "License" your contributions - do not "ASSIGN" your copyright to any project. It's a common technique used by tricksters to steal your intellectual property for their own diabolical, ulterior motives like you just described