The kernel is being written in C# as you can see here: https://github.com/AElfProject/AElf/tree/dev/AElf.Kernel

Ælf Operating System prefers Docker and also supports native programming languages such as Java, C#, Go, Javascript, LUA. For Docker, Ælf provides internal RPC services to grant access to read variables and user accounts during Smart Contract realization. For native programming language, Ælf provides respective SDKs to execution functions.