Their custom scripting language is extremely slow to the point where I wouldn't consider using it.
You can use C#, but if you download the c# version you get a warning on startup which states that this version isnt production ready.
You can use C++, but I found the workflow here awkward, plus they just introduced it with 3.0. They are about to launch 3.1 but this includes breaking changes to the way you use c++, so if you were using it and wanted to upgrade you'd have to rewrite a portion of your code.