Firstly - that was a MOVIE! FICTION! Her head spun around too if you recall. I hope you dont think that could really happen. Even the devil is limited by human physiology. It is physically impossible for a human head/meck to rotate 360 degrees. Secondly, tho God is all powerful, satan is powerful to a point itself and is allowed to mimic certain of Gods lesser powers. As an example read the bit in Exodus where Moses, with Gods power, goes head to head with the Pharaohs magicians. They were able to turn sticks into snakes and other stuff too, God just did it better and with more power. This is why we are warned to beware of false prophets in the end days. People will come who can work just enough 'miracles' (think Chris Angel but with a religious bent) to seem to be of God but who will really be of satan sent to lead us astray.
It isn't always, in fact most churches believe that tongues being a gift from God is something of the past, and doesn't happen anymore, thus in most cases it's a demon possessing the child (or adult depending on the situation) If you think about it that makes sense... seeing as demons have been around for thousands of years, you'd think they may know a number of different languages. Hope this helps!
First, it's a movie. Second, gifts given by God usually are not selective as to whether or not one is a Christian. Third, speaking in foreign languages is something lots of people can do. Just because someone picks it up instantly does not necessarily mean it's a gift from God.
alot of people act like they speak in tongues, but God wouldn't give a person the gift of tongues unless they needed, like if you went as a missionary to a foreign country and didn't understand their language, you might be able to miraculously speak their language and speak the word of God.