Yup its C# and then calling through to respective APIS for Directx 11/12/Vulcan etc and via xamarin for mobile. Coming from a C/C++ engine developer background I was very pleasantly suprised how performant Xenko is. It easily gives Unity a run for its money, C#7 is a nice scripting environment and the renderer is a thing of beauty both architecturally and in terms of output.
Its worth checking out.