Years ago the Church did believe this, although even to this day, some people still believe in being possessed by a demon or the devil and exorcisms were the cure. I believe they're placebos with dramatic flash to them, especially in the movies. The thing of speaking some other language that you don't know and need an interpreter to translate, I've never bought into because in the churches with all the high adrenaline, anyone can babble away claiming that it's the work of their god. Some others would buy into it and the translator would probably translate into things that the bible states already so it's nothing amazing. The odd thing though is when people say that the Holy Spirit or God will work through you and such, to me that sounds like possession because it's saying if you accept a supernatural being to guaruntee your arrival at heaven by doing certain things, that's exactly like demonic possession, only you're not going to heaven. Not all people who believe also accept this and I don't want to make it seem like all do but many do, even some people on
TH do.
To answer why a devil or demon would possess someone, you've got to look into occulate stuff, demonolatry and some polytheistic Satanism. I've looked into the last one for pure interest and it states that not all demons would possess people and not all are bad. There are some invocation passages to say, even for Satanism and some demons would have their own. As for why a random demon would randomly possess someone, that first requires belief demons exist (not a believer of that) and have to be a believer of possession.