The Linux Kernel is written in C programming language, so C is the most important language for the Linux Kernel developer. Initially, the kernel was written in GNU C (now it is also possible to build it using LLVM) which extends standard C with some additional keywords and attributes. I would recommend learning some modern C version like C11 and additionally learning GNU extensions to be able to read kernel code effectively.

Source: LXer – Core Knowledge That Modern Linux Kernel Developer Should Have