"There are thousands of volunteers who donated their labour to Duo... Bit by bit all of our work was hidden from us as Duolingo became a publicly-traded company."

Make sure to put anything you want public under the correct license. If a platform doesn't support CC or GPL or MIT, then leave.

EDIT: Or Apache, or IDGAF, of course. ;) But what I would really want is a license that forces your content to remain free, even if used in something else. Basically copyleft: https://www.gnu.org/licenses/copyleft.html