Tried. Done that. nothing to complain about. Was hard at beginning. But had NOTHING to do with language. It was hard because I didn’t know Slate and Editor. Didn’t know there is lots of problems are already solved, you just have to find right place and tool and create something new for editor.
Would different language fix it ? Of course not.As language have nothing to do with how API of engine is structured. You would just add another layer of complications, which would only make it harder (debugging problems, as you would still have to use C++ engine components, but then something break and you are stuck on interop layer when debugging).
Editor extensions, are still easy to write. But as with every new API, you have to first learn it, to use it efficiently.
Would I be able to write complex application in C# + WPF fast ? No. Because I don’t know WPF. That said making basic app in WPF is faster if you screw all the good practices while coding.
The same thing goes for Qt, wxWidgets and any other framework. Language is irrelevant in this case.