bai bai

Actually IS also the Japanese for "bye-bye". As anyone who speaks any language on Earth knows, there are a number of ways to say "goodbye" in every language -- and "sayonara" is but one in Japanese (and a pretty stuffy one, at that).

See ja.wikipedia.org/wiki/バイバイ for concrete evidence.

Hey everybody, let's not talk about things we don't understand!!

じゃあね、バイバイ！ (Jaa ne, bai bai!)