I know it's possible to mix C with Go, but I was wondering if it was possible to mix with other languages too. Specifically, I was wondering if it was possible to write a backend in Go and then use Swift or C# to make a GUI for a desktop app.
Also, is it common to build apps like that? What are the performance costs like? The goal is to write a cross-platform backend, then use native platform tools to create the GUI. So, I would use Swift and Cocoa to make a GUI for OS X and C# and .NET to make a GUI for Windows. That way the applications behave and feel like native apps.
Not in the same sense of using C/C++ with Go, no, however nothing is stopping you from writing the backend in Go and controlling it over http (or stdin/stdout).