EDIT The Go team announced in December 2013 that they will be transitioning the compiler to Go. As of February 2015, the compiler is exclusively self-hosting, as the C implementation was deleted. The new compiler shipped for the first time with Go 1.5.
Programming languages aren't programs, hence they're not "written" in any language. They are often described by formal grammars (e.g. BNF).
Interpreters and compilers for programming languages are programs and so must be written in some kind of programming language.
Go has at least two compilers, gc and gccgo. The former was written in C, but is now written in Go itself. While the latter is a gcc frontend written mainly in C++. Go's libraries are written in Go.