It should be under ZK_Component_Development_Essentials.
http://books.zkoss.org/wiki/ZK_Component_Development_Essentials

http://www.zkoss.org/forum/listComment/17409

And we should update the spec in WPD to let user know more details.
http://books.zkoss.org/wiki/ZK_Component_Development_Essentials/Creating_the_Configuration_Files/The_Widget_Package_Descriptor

<?xml version="1.0" encoding="UTF-8"?>
<package name="mymsg" language="xul/html" >
<script src="mymsg.js" /> <!-- set the default language when the specific language not found -->

<!-- Note the "*" do means locale actually .
for example , if your browser local is zh_TW , it does only map to mymsg_zh_TW.js -->
<script src="mymsg*.js" />

</package>