Anja Skrba, a Computer Science student from Belgrade, has done a Serbo-Croatian translation of at least some of the Java Web Parts documentation. Thank you Anja!
Take a look at this site for all kinds of framework tutorials, you will find the Java Web Parts AjaxParts tutorials under the Ajax submenu.
As part of the Java Web Parts project itself there is the Cookbook. The Cookbook contains a number of small projects using the AjaxParts Taglib. If there is anything you would like to see added there, please let us know.
An article (in German) about Ajax with Java and Java Web Parts.
An article from Frank W. Zammetti (the founder of Java Web Parts) about AjaxParts Taglib.
And ofcourse you can always read a book. Frank W. Zammetti (the founder of Java Web Parts) wrote a book about Ajax (Practical Ajax Projects with Java Technology, ISBN: 1-59059-695-1) which also contains chapters about Java Web Parts.