The love waltz (1930)

Originally Liebeswalzer, this German operetta was the third talkie vehicle for the effervescent Lillian Harvey. The plot is a typical Graustarkian affair, with Princess Eva (Harvey) preparing to marry a duke whom she's never met. Getting cold feet, the duke ducks the wedding, persuading a handsome young commoner named Bobby (Willy Fritsch) to take his place. The wedding goes on as planned, with Eva never suspecting that her new hubby is a ringer. Eventually, the false duke confesses everything, leading to all sorts of intrigue before a happy ending can be realized. Love Waltz was simultaneously filmed in an English-language version, which posed no problem for the British-born Harvey but caused a few uncomfortable moments for her Teutonic co-stars (eventually, Willy Fritsch was replaced by John Batton, who'd played a bit role in the German version).