Sphinx is a tool that makes it easy to create intelligent and beautiful documentation, written by Georg Brandl and licensed under the BSD license.
It was originally created to translate the new Python documentation, and it has excellent support for the documentation of Python projects, but other documents can be written with it too. Of course, this site is also created from reStructuredText sources using Sphinx!
The Django documentation on internationalization describes how to add multi-language support to your application. As it took me a few tries to get it right, here's a rundown of what I did to add it to an existing project.