Duolingo might be in violation of your country's regulation against "Gacha games" (AKA games which feature loot-boxes), but I need help figuring out if that's the case because I can't see whether the in-app activity is available to players in countries where such games are regulated.
Duolingo regularly issues "quests" to its users, in which they complete daily and multi-day quests to earn points. If you complete the target number of quests for a month then you earn that month's badge. As far as I know there are no regulatory risks to this sort of reward mechanic.
The problem is that they've also started doing other kinds of side quests, with the most recent one being Junior's Toy Hunt. For this minigame, users complete lessons for a chance to draw a random puzzle piece, with a maximum of four pieces issued per day. Users that gather the full set of nine pieces from one of six sets are rewarded with a cosmetic suit corresponding to the set they completed.
Pieces are drawn randomly, and five duplicate pieces are automatically traded for a new piece. Not all pieces are equally likely, but I've assumed they are and estimated that on average you should be able to get all the sets by drawing 108 pieces. At four pieces per day, this should take you 27 days. This should be achievable for consistent players, but it's not guaranteed. If my math is correct, the (incredibly unlikely) worst case scenario is that it will take you 266 piece draws, or 44 days, by which point the challenge will have ended.
The problem is that you can also spend "Gems" (Duolingo's in-game currency) to buy additional pieces at random. You can buy one piece for 400 gems, or five for 1600. The app issues gems as rewards for completing challenges, but you can also just buy them (1200 for €4.99, 3000 for €9.99, 6500 for €19.99). This means you can pay for a random chance to win in-game cosmetics, which is where I think they're starting to cross over into potentially violating various laws which are mostly intended to prevent minors from developing gambling habits.
I found a table of Gacha regulations by country on the Gacha wiki. If you live in one of the countries or economic regions listed on the table, have Duolingo installed, and can confirm whether this mini-game is available to you, please respond indicating your findings.
For the record, this is unregulated where I live, so I have no intent of following up on this with a legal case. I'm mostly just curious about whether they've screwed up by launching this feature. That said, if someone else does want to follow up on this in some meaningful way, please let me know and I can share my estimates of how the probabilities work out.