Personally I would go with XML instead of a Java Resource Bundle. Self-explanatory, any one with basic HTML knowledge can change the properties - think about the folks in the field. The moment things are Java'ish, people are a bit more hands-off - its programming I don't understand all that stuff. This ResourceBundle thingy sounds more like Java Programming rather than a simple text file (assuming that's how you are implementing, hard to say).
Now that the JDK has support for XML, you don't need a lot of third-party tools for using it. More easy to maintain, IMO. This sounds more like the norm - look at any config files that application use - they are invariably in XML so its pretty much an industry standard.