This doesn't answer your question directly, but Struts has internationalization (i18n) built in. In a nutshell, it will get the locale and language of the browser making the requests and use the corresponding application resource file. It works like a charm.
I believe it is possible to load the app resources at will, but I think it opens up a whole ton of synchronization problems. I've read a bunch of threads where people try to dynamically load an app resources file to no avail.