Up to now, the main language in which KDE applications are developed is C++. This is a perfectly valid choice: however, at least for users that don’t have a computing background (I include myself in the list: I do science) C++ can be a rather steep requirement (although Hans, fellow staff member at the KDE Community forums, is doing a nice job in explaining his learning experience). Scripted language bindings (that have a lower barrier of entry) for KDE exist: on top of my memory I can list Python, Ruby, and C# (and I’ve probably missed a few).