[Boogieman]

Why do you think that? Christ and the Apostles didn’t use Latin when they cast out demons, and they were still effective.

[Campion]

The rite could be in any language; demons understand them all just fine. The only reason for putting it the vernacular is so laypeople present can understand it. Why would anyone think they need to understand it? Only if they don't really believe there's anyone else listening!

As I explained above, the prayers aren't addressing people and aren't there to teach people. They're directed to God, and in some cases directly imprecating the demons in God's name -- in at least one case, at some length. (Several pages worth.) Real demons understand them just fine as they are.