What is the best programming language for writing MIDI-interactive musical application that would be run on MAC and on Windows?
I've written cross platform (Mac OS X and Windows) MIDI applications using C++. I used the RtMidi for MIDI I/O which was incredibly easy to get going (on every platform just it just worked out of the box). The advantage of C++ is that you can use native GUIs on each platform, but if you're not already into C++ I'd say a cross platform project is a pretty ambitious first project.