I'd love to watch that one too. Been on my list for a while. It's the first part of two in an adaptation of Die Nibelungen, apparently. The second one is called Kriemhild's Revenge.
I've been thinking of exploring Lang closer this year. I've only seen his most famous ones (along with the Dr Mabuse trilogy), but they're all great. Apparently he himself considered M to be his best work so you're in good company. You really need to see the extended version of Metropolis though. It's a masterpiece in its own right.