I haven’t used JavaFx so I can’t answer that part of your question but XAML has been cross platform with Xamarin on macOS and Android and now everywhere with MA

I haven’t used JavaFx so I can’t answer that part of your question but XAML has been cross platform with Xamarin on macOS and Android and now everywhere with MAUI. I also mentioned Uno which lets you develop for web/electron with XAML. Language wise, it has been largely C# or other .NET like VB.NET, F#, or managed C++ though I think Noesis lets you use it from C++ and other languages directly. C# itself is completely uncoupled from Windows ever since .NET Core and now with later .NET versions like .NET 8 all Windows-specific bits have been torn out altogether. Microsoft probably uses C# on Linux more than they do on Windows these days.