Day 10 report: 3x15min sessions each day
All-in-all it seems that there are ways to organize such a challenge better. Reporting in only twice means that most of the people manage to forget about it already by day 5. Also I am somewhat surprised none of the Lingvist team members, who decided to take part, didn't report in halfway through the challenge. I'd expect you to be the ones most interested in keeping the challenge alive and setting a good example to other participants.
In your position I would think of creating something like a (semifrequent) week-long challenge for reaching daily 50 cards or smth (no big target, just to build microhabits of learning a language and doing it via Lingvist). People sign up, their progress gets tracked automatically and everybody can see on a challenge page what percentage of people are still in. In case you haven't done your cards for a certain time of a day you get a simple reminder e-mail as long as you are still in the competition, once you fail your target for a day, you are out of the challenge.