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This is a known bug in Maven. The problem is that you likely have a 'dspace-xmlui-lang' or 'dspace-api-lang' folder at the same level as your [dspace-source] parent folder. Essentially, Maven located them and tried to add them into the build process (which it shouldn't have). The fix is to completely delete the "dspace-xmlui-lang" and "dspace-api-lang" folders, and try to rebuild DSpace.

Advice for future Release Coordinators

With a straight face assure the next Release Coordinator that "Maven is easy" and there is nothing to be afraid of, then put your feet up and open a beer.