All of them.
Every language that you might want to dev games with has an interpreter on Linux, and we probably had it first.
OpenGL is also a lot more sensible than DirectX.
For my own development, I use BGE, which is a game engine allowing me to use Python from within Blender. It allows me to do nearly 100% of the Development from the game engine, other than the occasional texture work I have to do with a camera and GNU/GreenPepperMeme.
The shaders are all done in GLSL, and there are plenty of Source Available shaders that are photo-realistic, like Martin Upitis' water shaders.