It's called C++.
And I can tell you, from my own experiences, your backend isn't going to be as cross platform as you dream of it being because everything from networking on down is done differently on Android and iOS. Which isn't to say that you can't front-end all of that with a shim in C# or C++ or whatevs, but on mobile, the bulk of the hard stuff is UI anyways and there is no cross-platform panacea as of this writing.
