I would like to add that the main issue with a goat being able to speak English is the hard palate and teeth of the goat's mouth, and not the vocal cords. Goats are very loud, and make bleating, honking sounds like a whiny human children very often (I suspect they can tell it bothers humans so they do it more). In the lore of most demonic stories, demon possession changes the physical laws that normal constrain humans (as described by @Gilles) so I think that the demon could pretty easily force the palate into a shape to form English words.
Two other notes; in Christian lore, Satan is often depicted as taking a goat form, so the goat may have briefly become Satan himself. Additionally, the demon may just be manipulating the experiences of the humans around it, and psychically implanting the idea that the goat spoke in English directly into their minds.