When you create your XAYA game with Unreal Engine or Unity, you have options when deciding how to program your game. Recall the basic architecture of a game built on XAYA:

The red libxayagame and xayad are "black box" components that simply do work for you.

The green game logic and front end are what you need to program yourself. This is all your game code, both back end and front end.

The front end is done in Unreal or Unity, so you'll be writing code for that as normal.

However, the game logic portion can be in any language, so you can write this in C++ or C# or even Haskell or Erlang if you so choose. The choice is yours.