I was also facing this problem for Struts2 web application. Please use following steps to solve your problem
1) The solution will be to add the displaytag.properties to the path where ApplicationResource.properties is saved
2) displaytag.properties should have the below lines
3) Copy the below code to a new java class "I18nStruts2Adapter" at "com.web.displaytag.resourceslookup"(package path can be change as per your package structure) in your project.
Hope this will helps to other struts2 user who are facing this problem. Happy Coding !!!