Godot has three kind of languages integration :

- GDScript : included by default

- C# : you have to download the Mono version of Godot (and install the Mono SDK)

- Any other language (Rust, Python, C++, D, etc...) : using GDNative

(- And of course you can also directly modify the engine source code in C++ if you need super low level modifications (like writing your own render engine))