"A desktop environment does not need its own language ..." I'm unsure that I agree. Given that part of Vala's raison d'être was interoperability with C and that it integrated well with the parts of the desktop environment that made it a desktop environment (i.e., the gadget toolkit and the accompanying services), if I wrote GNOME (Gnome these days?) applications, I'd probably be fine writing the UI in Vala and the rest of my logic in something that, at a minimum, spoke the C ABI.

Likewise, I used to write Windows software in Pascal with parts in C and C++ because I oddly both found Win32 and COM and found interfacing with third-party libraries easier to deal with that way.