Saying that your locale is "jp" is not really valid, you either specify your language OR you specify your language/country (eg either "ja" or "ja_JP) but never just the country as some countries speak multiple languages.
en_GB and en_US will both default to "en" if a more specific message bundle can not be found Tapestry uses java's ResourceBundle under the hood ( http://docs.oracle.com/javase/1.4.2/docs/api/java/util/ResourceBundle.html)
