I ran into a similar problem
I use an Spring/Struts application
1)the solution will be to add the Displaytag.properties to classpath say if you use an
IDE add it to Src directory or if you are using a build script make it to fall in the classes directory
2) Displaytag.properties should have this entry
Struts: (say you are using a Applicationresources.properties)
locale.provider=org.displaytag.localization.I18nStrutsAdapter
similar way for other frameworks
Hope this helps others too who are facing this