What parts are written in Lua language?
Or
for what parts the Lua language is used?
Or
I am a Gulang programmer
I can use this source For logic my games
@Mahdihp I recommend you read the documentation for the game backend and multiplayer engine because it will answer a lot of your questions 
What parts are written in Lua language?
The gametech is written in Go as you can see on GitHub. We have a server framework which lets game studios and developers write their own code in Lua, TypeScript, or Go. The custom code you write will be loaded by the game backend at server startup.
for what parts the Lua language is used?
The Lua language is one of the supported server framework languages that you can write your custom code in.
I am a Gulang programmer
I can use this source For logic my games
I think you meant Go programmer. You don’t fork the codebase with Nakama. The way to would write your own Go code and use it with Nakama is described here.
Hope this helps.