WebWork 2 supports internationalization (in short, i18n) in two different places: the UI tags and the action/field error messages.
TODO: Describe or point to how to add i18n to an application using ww2
Using a master application catalog
Struts users should be familiar with the application.properties resource bundle, where you can put all the messages in the application that are going to be translated. WebWork2, though, splits the resource bundles per action or model class, and you may end up with duplicated messages in those resource bundles. A quick fix for that is to create a file called ActionSupport.properties in com/opensymphony/xwork and put it on your classpath. This will only work well if all your actions subclass ActionSupport.
If this is not the case, another solution is to create a ServletContextListener and have it call LocalizedTextUtil.addDefaultResourceBundle(String resourceBundleName). You can also use a Servlet that's initialized on startup to do that.