Which is the best skill for developing games in sololearn?

Answer 1

For games with graphics in solo you are limited to web only.
i.e. html, css, js

Answer 2

depends on what kind of games. I use c# for 3D games. you can also learn c++

Answer 3

If you mean which language to learn in generell (which is available in Sololearn) I would say C++ and also maybe Python.
However it depends on the kind of game and you are more or less free to choose your favorite language.