what programming language is used to create slender?

I never played the game, but i just downloaded it to see how it's done ;)

The answer is: They mixed UnityScript and C# scripts almost 50 - 50. However it seems they included a lot scripts that aren't even used.

Also it's a pain to see another Unity game where the developers almost exclusively use FixedUpdate for everything... Please, don't do that. It causes a lot (really a lot) problems. Especially Input detection. Movement and rotation in FixedUpdate will look and feel even more choppy. Generally do everything in Update that doesn't use forces on rigidbodies.

Probably C#, but the same can be done with the other Unity coding lanuages.