Not sure how old this post is but...THANK YOU for clearing up this decades-old mystery! There's a clip on Youtube that proves conclusively that it is indeed the Yemano gaku ka song: https://www.youtube.com/watch?v=L06HFBrNhXk
Why the claim of a German folk song? I chalk it up to pure marketing. No one knows what Yama No Onkika is. Also, back in the 80's it would have been far more acceptable in America to call something German than Japanese (remember the term "Oriental"?) So cover it up with a moniker that people can swallow, and boom, you're rich. Brilliant move, Casio. And brilliant detective work to the OP!
