I think C and C++ are the most popular languages for hard real time development, so you'll have better OS support, better best practices, more experience if you choose one of those.
But I don't do things in hard real time, so please correct me if I'm wrong.
A lot of aerospace real-time environments are still geared for Ada, and while C/C++ are available they are not the same as "vanilla" C/C++. Also, is there really C-specific support in a good RTOS? Probably doubtful, except for inertia or presence of header files if they expose them.
But I don't do things in hard real time, so please correct me if I'm wrong.