As found in the documentation:

Unity supports three programming languages natively:

• C# (pronounced C-sharp), an industry-standard language similar to Java or C++;

• UnityScript, a language designed specifically for use with

Unity and modelled after JavaScript;

• Boo, a .NET language with similar

syntax to Python.

As found in the documentation, there is one "native" choice:

• C# (pronounced C-sharp), an industry-standard language similar to Java

Additionally, many other .NET compatible languages (like C++) can be used, if they are first compiled into a DLL (then you'd have to write wrapper code in C#).