>Also I don't get it why they use so many languages at once (Java, Kotlin, Groovy) and I think some parts are in C++
They seem to be mainly a Java shop. They probably use C++ for some performance intensive tasks where it makes a difference.
Since they are creating Kotlin, they are starting to use it for their codebase as well.
Their different IDEs seem to be built on the same java core, but with extensions in the target language : Python for PyCharm, c# for ReSharper, ....
Their different IDEs seem to be built on the same java core, but with extensions in the target language : Python for PyCharm, c# for ReSharper, ....