You've misunderstood how to define resource files. The name you define in the struts-config.xml file represents a bundle or group of files. The group is represented by a name without a language suffix.
Struts will then append the appropriate language suffix when it determines which language the browser is set to use. So, your example,
you should rename the file ApplicationResource.properties to Application.properties and change your struts-config.xml file so it has only the following entry for message resources:
Struts will then check the language of the browser and look for a file with the appropriate language suffix (e.g. Application_en.properties) and if it doesn't find that file it will use the default file of Application.properties.