I would guess part of it is if they have a FA on board that speaks the other language. From the AMS flight I could see not having a Dutch speaking FA and it would not make sense to do the announcements in French or German on a plane that has a lot of Dutch people on board. But flights to countries where the language is German, French, Japanese or Chinese is probably going to get the bilingual announcements as there are FA's on board those flights that speak those languages, usually.