[mikeash]

I think C# is pretty a pretty common cross-platform language for games due to Unity's support for it. It's not common for other apps, though.

I don't think there's much cross-platform work being done in Swift yet. Give it a little time, though, it's only been open sourced for a few months.

One nice thing about it compared to many other languages is that it can use C APIs with almost no work. There's no need to screw around with annoying FFI systems, you just point the compiler at your C headers and the calls become available in Swift. They may not be very idiomatic Swift, but they're at least no more difficult to use in Swift than they are in C. That means that you don't necessarily need a bunch of Swift libraries to be built before you can start getting stuff done.