Well if you want to use C# you could look into
Mono[
^], that should allow you to run .NET applications on Linux as well (I haven't tried it personally). But really you can use any language you want, Java might be a good choice, there are multiple IDEs that will work on both Windows and Linux (Netbeans and Ecplise come to mind, and I'm pretty sure Netbeans has a visual form builder similar to Visual Studio's)