Let's hope Unity will support Rust, too, then...and I guess Go now as well, if Google says they want it for games.

Likely not - C# seems here to stay on Unity3D, but speeding up execution through tools like IL2CPP seem most likely.

http://blogs.unity3d.com/2014/05/20/the-future-of-scripting-...

Unity only supports managed code unless you have Unity Pro, and its not clear to me how well you can use other languages; I think the support may be more for middleware libraries and such.

Would they have to be ported to Mono to be used for Unity?

They don't even support the latest version of C#.