It's pretty much language-independent (and therefor python is the best choice
)--what's more important is what widget-kit you use (GTK, QT, wxWindows/-Widgets/-Python, xaw3d, TKinter, ...). I think GTK hits the best balance of portability (dos, *nix & (iirc) old-mac), looking-good-'ity and ease-of-programming-'ity.
hth --Jonas