@Felthry Hmm, I'm trying to figure this out now — the Japanese title does indeed say バテン・カイトス so I've always pronounced it that way myself and never thought about it any further?
The Wikipedia page for the star says /ˈbeɪtən ˈkeɪtɒs/ but I don't know if they have a source for that; there's two references nearby but one of them doesn't cover pronunciation and the other is a book so I can't easily check. Googling various permutations of "baten kaitos pronunciation" isn't helpful either; there's a lot of people trying to find official word from Namco as if the title was invented for the game and not the name of a star. I dunno where to go from here...
@Felthry I don't. Google Translate's automatic pronunciation of بطن قيطس is wildly different though lol