It's based on [link|http://wxwindows.org/|wxWindows].
Cross-platform:
wxWindows allows you to target Windows 95/98/ME, Windows NT/2K/XP, Linux/Unix with the GTK+ toolkit (or plain X11, or Motif), and MacOS.
[link|http://wxwindows.org/develop2.htm|Cross-language]. There are bindings for at least 10 languages that I counted quickly on that page.