What language are the shaders in Blender written in? What language are the shaders in Blender written in? Like the Principled BSDF and the rest.
Most of them are written in OSL, OpenShadingLanguage, a language specifically designed for shaders. It's similar to C and can be compiled, so that it's fast.

You can find it in the Blender sources at

intern\cycles\kernel\shaders\node_principled_bsdf.osl


OSL is comparable to RSL, Renderman Shading Language and GLSL, OpenGL Shader Language.