Although C# should be a much easier language to deal with, so this one might turn out better.
(I love my ReSharper Ultimate license, and R#++ helps to fill in a lot of Visual Studio's blanks as far as C++ goes, but really it needs to spend another six months incubating.)
I doubt they use all the languages their IDEs support internally - there's just too many! (Java, PHP, Javascript, Python, C/C++, Ruby, Obj-C). The problems are probably because CLion is one of their least mature products.
The notable exception is anything based on clang, which is usually flawless (as you would expect). If CLion had been based on clang then they would have saved themselves a lot of work and ended up with a better performing product. Presumably the reason they didn't had something to do with licensing.