Pootle is a simple web portal that should allow you to translate! Since Pootle is Free Software, you can download it and run your own copy if you like. You can also help participate in the development in many ways (you don't have to be able to program).

The Pootle project itself is hosted at translate.sourceforge.net where you can find the details about source code, mailing lists etc.

The name stands for PO-based Online Translation / Localization Engine, but you may need to read this.

This site is running:
Pootle 1.3-svn
Translate Toolkit 1.2.0
jToolkit 0.7.8
Kid 0.9.6
ElementTree 1.2.6
Python 2.5.2 (r252:60911, Jan 4 2009, 21:59:32) [GCC 4.3.2] (on linux2/posix)