Pootle is a user-friendly web application for online translation of contents. Its used by organizations like Creative Commons, OpenOffice.org and GNU/Linux Matters.
Read it here
Im going to show you how to install it the easiest way, from my experience with the GLM Translation Service under Debian 4.0 (this guide might also work under Ubuntu, though). Ill use the sudo utility, so I assume that youve already installed and configured sudo accordingly for your user.