Currently, while installing the IdP, administrators need to adapt various messages in multiple messages properties files to adapt the messages to their needs. During future upgrades, they need to take care to keep the adaptations in case they want to update the messages properties files. This might be a cumbersome task. If administrators are using translations provided from other people, e.g. those provided at https://wiki.shibboleth.net/confluence/display/IDP30/MessagesTranslation, they may need to add their local adaptations to these files, too, in case they want to update these files with new versions.
It would be convenient for an IdP administrator to be able to manage adapted message strings at a central place, e.g. messages/local-messages.properties, instead of changing the messages spread over multiple files. This would make it easier to update the other messages properties files.
I propose to add a further messages properties file, e.g. messages/local-messages.properties, which is initially empty. This file should be listed as the first one in the "shibboleth.MessageSourceResources" in conf/services.xml, so that messages defined in local-messages.properties override other messages having the same key.
Furthermore, if messages/local-messages.properties initially exists and is included in services.xml, it's a convenient place for administrator to put their custom messages.
I have attached a patch that adds this file to the configuration.