Elementary wants to be “the Apple of Linux”, to attract less-technical users and switchers from other OSes. Their development philosophy is similarly top-down: someone sets a “vision” and a small group of people gets to implement it. If you are a coder and you value bottom-up development, you should really look elsewhere.
Plus, most people don’t know most languages, so no matter what you choose, most devs have a language barrier. Vala, again, is very easy to pick up — certainly easier than C or C++ and probably easier than Java/C#/D.
New apps in the Pantheon system could I guess be written in any language.
GCC is stuck on a pseudo Objective-C 2.0 support.
clang does support the basic stuff, but there are no libraries.
Yeah, there is GNUstep, but it is like stuck on Pather level compatibility or something like that.