Although Jaxe has been localised to English, most of the source code is using French, and most comments, variables, methods, and classes are in French. So, knowing French can be useful to develop with Jaxe, but it is not necessary in most cases, since you can just follow the following recipes. Also, most CVS update comments on sourceforge are in English, so it's also useful to know English. But the language you really have to know is of course Java !
This documentation explains how to use Jaxe within another application, and how to add plugins to display elements.