In an email, a Bookcase user asked me about translated field titles and mentioned that they didn't change when Bookcase was run with a different language. The reason for that is just that the field titles are directly saved in the data file. Once a new collection is created, then there's no easy way to know which fields should continue to be translated without explicitly asking the user, and I considered that to be too intrusive. So if you have your collection saved with field titles in French, and send it to your German friend, he'll see French.
WIth that said, there is a way to revert to default field titles in the current language. Just create a new empty collection, and import your data file, choosing to append it to the current collection. That will revert all the default field titles to the current language without changing anything else, I think.
Heh, as always, if you care about your data at all, back it up.