Answer 1

For GameDev, one language wont cut it; as it will limit yourself.
But if one language does help, it will be C++ due to the fact it has everything you need to know about programming languages.
Knowing and mastering C++ will make learning new langs a breeze.

However, sticking to C++ only will restrict you from using Unity, the most popular and used engine/environment in za warudo! uses c#. You cant make web games, JS.
Android or IOS (Java or Unity and whatever IOS uses).
Console games (used to be c/c++ and similar but could have changed. Though Unity can export to console now).

If youre going professionally, learn C/C++, C#, JS, Java, Lua, Python, and learn graphics (OpenGL, Vulkan, DirectX)