I believe the Bible is number one. I certainly has been translated into more languages. It won't be represented in too many bestseller lists because EVERYONE has one already.
I have the boxed set of HP 1-4 (in Russian). I was trying to learn Russian at the time. You can't imagine how funny Dursley is in Cyrillic.
DK
Or more than one. My daughter needs multiple translations for a religion class, so my wife and I searched the house tonight. We found six English translations and one Spanish. That's not counting the NIV my daughter has with her.