Zircon is the core platform that powers the Fuchsia OS. Zircon is composed of a microkernel (source in kernel/…) as well as a small set of userspace services, drivers, and libraries (source in system/…) necessary for the system to boot, talk to hardware, load userspace processes and run them, etc. Fuchsia builds a much larger OS on top of this foundation.
Google changed the name for this project from Magenta to Zircon, which seems like an opportune time to highlight it.
Taken from golang-dev mailing list discussion.
There are other modules implemented in Go, like the file system daemon.
Other major language being used are C++, Rust and Dart.
And the surviving Magenta parts still written in C are being rewritten in C++, as per Git logs and wiki.