If you follow typical Java SE and Java EE internationalization practices, you should be OK. As an example, one of my apps uses the message tag in the fmt JSTL. The messages are in xxx.properties. If I change the language of my browser to German, then the text from xxx_de.properties is picked up. I have also used the internationalization support offered by frameworks such as Struts, WebWork, and Tapestry.
Note that this is not a JBoss issue.