Currently the keys for the I18N strings are of varrying formats and "standards". Most follow the pseudo-standard of <area>.<content>.<level> where <level> is one of "info", "err", "notice", <area> is a short hand for user of message (ie DefaultInfoBuider may use "builder") and <content> is the id of specific message such as "missing-header".
However this is not always followed and can be hard to maintain. It would be better to go to a more formalized/structured key space such as;
MSG-0001 : message used when displaying interface to users
ERR-0001 : error message representing failure
WRN-0001 : warning message that represented a failure that is guarded against and does not halkt the process
This would make it much easier to verify that all the messages in source code have coresponding entrys in resource bundles using simple perl pattern matching script.