You can even directly test your localization resources without having to change your system localization!
To get started, create a new Widgetarium project, drag your source, html, style sheet, and image files to the project, and run (or start from one of the basic templates). If you've already got a widget you're working on, you can create a new project by importing the sources and structure directly from that widget. Widgetarium builds your plist for you (which you can adjust explicitly), and copies all the files into the resulting ".wdgt" file, all ready to be installed in Dashboard.
For questions/support, please check the Web board.