What language is it? Sounds like gibberish with British accent, but a few words seem to be Latin or even French.
Sounds like gibberish
It's just a indication that the words aren't important to the action....it's just a general noise going on in the background.
Having actual words being discernible would tend to concentrate the audience on the dialogue which isn't the point.
...the vicar (voiced by Rowan Atkinson off-camera) gives his apparently gibberish sermon
Plus it means you don't actually have to write them!