What languages are most AAA games written in and why?

Objective answer: C++, better performance.

But in truth, there are often many different languages used in the same project, each for a specific part of the project.

They're not written in languages, they're written in engines.

And what is engine based on? It depends. Unity for instance is based on C#, unreal engine on C++. Other may be based on even other things.

C++ - Potentially best performance, very good multi-platform support, very compatible, universal, widely used.
C# - Much easier, faster to code complex parts of the code, compatible mostly with Windows only (although microsoft is working on making .NET open-source, so it may change soon)

There's also Java. Java is very similar to C# in terms of how it works, but has much better support for multi-platforms. It's fairly slow though, and C# is superior for today.

And those are only base languages. Engine is combined of many more of them for certain part of the codes. For example, performance-critical parts such as rendering might be coded in C++ purely for performance and memory management, even though whole engine is based strongly on C#. This is the case with Unity.

You do not learn "language" nowadays to make games. You learn engine, e.g. Unity, and during doing so, you also learn the base language it uses, C# in this case.

English

har, har har.

assembler xD

As a student who recently got into Unity, I learned that most of the programming I do doesn't require language X or language Y. The most essential is to get better in thinking the logic of what you want to create.