Unity supports C#, a flavor of Python and Java via AOTC. (That's ahead-of-time-compilation.) External plugins are likely to be written in Mono C#.
"C" is a low level language usually used for systems programming, it doesn't support classes or objects or even strings natively. You wouldn't write a game in C unless you were a masochistic coder or you planned to run on bare metal. (Without an operating system.) "C++" is for the most part used as a library language, it's great for things like 3D engines and OS boiler plate code. Because of the manual memory management in C++, I suspect it's used more often in console games. The newer and higher level languages are better for applications as they allow rapid iteration on code an have expansive libraries for things like image compression, XML, modeling, codecs, etc.
It's not 1992 anymore and things have evolved past C/C++.
If I remember correctly they said they would stick to C# for the Unity programming.
Am a leedle bit sad that that means no Boo (just for the coolness factor of Hamstercode)