The language is C# in the unity engine, I've seen some people decompile the game using dnSpy but it can bee tricky as it doesn't perfectly give you the source code. I haven't open sourced the game but I'm considering doing so in the future. Whether I do or not, I want to add modding support through something like .lua scripting and 3D model importing, and create tools to allow you to make your own weapons/monsters/areas.
Unfortunately I'm slooooow T_T but we'll get there.