Anytime you think
runtime modificationin a Java EE application, you should think database (or more generally, data source). There are tricks for using file system read-write resources, but it is generally discouraged.
Here is how to put message bundles in the database: Storing your messages in a database
If you really are looking for a
lightweightdatabase, then use an in-memory database like H2 just for your resource bundles.
Thanks for the hint, Dan!