That's what the character in the movie believed too... he explained it away using science. That the possessed were really schizo and that the speaking of English (she was native Italian) and other languages were due to the person's age and demographic. If there is one thing that I've learned in corporate america, is that you can explain anything away, even hard facts and numbers.
I either case, convincing anyone to do or be something else when that person is already resolved to do the other is pretty fruitless. Resolved being the keyword there.