Language Definition
From Documentation
A language definition defines a component set (aka., a language). For example, ZUL and XHTML are two component sets.
To define a language definition, you have to prepare a file called /metainfo/zk/lang.xml
and makes it available to the classpath (such as in a JAR file, or in WEB-INF/classes of a Web application).
A language addon is used to extend a language definition. It shall be called /metainfo/zk/lang-addon.xml
available to the classpath, or specified in WEB-INF/zk.xml.
When ZK starts, it will parse all language definitions and then all language addons based on their dependency. A language addon is a variant of a language definition. They are almost the same, except the naming and it must specify the addon name.