This plugin is pretty fucked up... :-/
I've had a quick look. I translated the holiday day to French. Added Bastille Day for good measure (it's French software, FFS :P), went to the admin, tried to add the day... Hmm, nope.
Uninstalled plugin. Told Wedge to delete all data (i.e. calendar tables). Then reinstalled it. Guess what..? The holiday list is completely empty. -_-
There are indeed untranslated strings, notably when 'adding a new holiday'.
Plus, whatever's mentioned above, and that I'm SURE is correct...
What I'm not sure of, is whether I want to get to work on this right now, uh...