I wonder: Is this because quid quid latine dictum sit, altum videtur?
Or, do you have some sort of sneakiness planned because of Rowling's use of pseudolatin for magical formulae and/or for prophecies? If so, I have a few guesses based on the current storyline, but that only makes me want to see what you have planned all the more.
I'm keeping an eye on you, EY. (But only because I've greatly enjoyed reading through HPMOR, and am eagerly awaiting the next update.)
It's because Latin was the default language of scholarship and 'high' literature in 13th century Muggle England, and given the importance of (mangled) Latin for spells this would presumably hold true for wizards as well.
Also, if the piece is from Roger Bacon's diary, he definitely wrote in Latin.
If anyone can do non-wrong Latin, I could use a translation of the following for HPMOR. The original is supposed to be circa 1200.
No rescuer hath the rescuer.
No Lord hath the champion,
no mother and no father,
only nothingness above.