I prepared three manuals for translation today: German, Spanish and Portuguese. You'll find the links for them at the bottom of this mail. So how can you help?
First, go to the page and create an account for booki (if you don't have one). Then you can login and see the chapters of the book, initially an english copy. When you click "edit" on a chapter, you can start writing right away in your browser. So you are replacing the english text with your own language. Don't forget to click "Save" before you leave the window!
You should always leave the window (using save) when you stop writing, because only one person at a time can edit a chapter and it would be blocked if you don't leave.
Please let me know if you have more questions and also if you would like to see more languages!