You're right, Susanne. As is pointed out in another thread, TBTM incorporates some musical phrases from "Die Lorelei," but it is certainly not the same tune.

Someone had told me that "Tomorrow Belongs To Me" had been "the theme song of Hitler Youth," but that was one of those 'urban legends." Yes, it was composed for the musical "Cabaret."   

I still would't feel good about singing it, even though I know it's just a "show tune," because it does have Nazi associations.