The allowed child elements include addon-uri. You might have multiple language-config elements in one zk.xml.
<language-config>
<addon-uri>/WEB-INF/lang-addon.xml</addon-uri>
<addon-uri>/WEB-INF/lang-addon2.xml</addon-uri>
</language-config>
Note: Unlike most other configurations defined in WEB-INF/zk.xml, the definitions defined in language addons are applied to all Web applications sharing the same zk.jar.
In other words, the definitions in language addons are visible to all Web applications sharing the same zk.jar. Furthermore, it may cause errors in another Web application, if the classes or resources are available only in the Web application defining this.
Thus, if it is an issue, just put zk.jar and relevant ZK libraries under the WEB-INF/lib directory.
[Default: none]
It specifies the URI of language add-on definitions. To specify more than one URIs, you have to define them with multiple addon-uri.
A language addon is used to add new components and override the definitions of existent components. Refer to the Component Development Guide.