Clone the source, then create a directory corresponding to the target language’s IETF tag: see the W3C article on internationalization. For example, English is "en", Japanese is "ja", and Traditional Chinese is "zh-Hant". In the new directory, and translate the txt files from the "en" subdirectory.

For instance, to translate the guide into Klingon, you might type:

$ git clone git://repo.or.cz/gitmagic.git
$ cd gitmagic
$ mkdir tlh  # "tlh" is the IETF language code for Klingon.
$ cd tlh
$ cp ../en/intro.txt .
$ edit intro.txt  # Translate the file.

and so on for each text file. You can review your work incrementally:

$ make LANG=tlh
$ firefox book.html

Commit your changes often, then let me know when they’re ready. GitHub.com has an interface that facilitates this: fork the "gitmagic" project, push your changes, then ask me to merge.

I like to have translations follow the above scheme so my scripts can produce HTML and PDF versions. Also, it conveniently keeps all the translations in the official repository. But please do whatever suits you best: for example, the Chinese translators used Google Docs. I’m happy as long as your work enables more people to access my work.