Also collective.themesitesetup registered translation overrides TTW, though it only UI was theme editor and changes were applied only when theme was "updated" or "activated". I used zope.app.i18n, which used to provide persistent message catalogs. I haven't looked into collective.ttwpo yet in detail, but with zope.app.i18n I had no problems in registering new catalogs or appending / overriding msgids in existing ones. (Only exception is JS "widgets" domain, which does not support cascading catalogs and therefore cannot be overridden, but instead it's ok to register new JS translation domains.)