Answer 1

To participate in a challenge 1.click on the challange tab in android app
2.click on th '+' icon to create a new challenge
3.then choose the language in which you want to challange some one
4.select opponents will be next option either you can challenge any random opponent or the suggestions given by sololearn or even your followers or people you follow.
5.then you could complete your part of the challange and wait for the opponent to accept your challenge and if he/she accepts it and completes it .
6. The winner will be awarded and loser have to payup in compensation in 'xp'.
Extra info : challange some one in someting you are having knowledge for that dont skip your classes any way all the best!

Answer 2

You can find many active challengers of different languages in the following thread!
Just comment your expected language and wait for the challenges.😉😉

https://www.sololearn.com/discuss/864805/?ref=app