Odd then that he speaks it in The Exorcist and Father Karras exorcises using English.
You'd think the director might have wanted to have the priest say the exorcism in Latin rather than English to suggest his authority over the demon. But maybe the director wasn't too hot at Latin at school or something, and he wanted to tell everyone what a diabolical language he thought it was.I think Father Karras' attempts in English are a bit of poetic licence on the part of the author/director. The English translation of the Rite of Exorcism didn't exist at that time. It was the last of the rites to be revised following Vatican II, and the approved English translation is very recent.
Latin seemed generally in decline in the '70s, and probably before that.