The production designer for Die Nibelungen was the enormously creative Otto Hunte (b. 1881 d. 1960).
In this photograph he is seen standing at far right, next to Fritz Lang. He was also in charge of production design for Metropolis, Der blaue Engel, Die drei von der Tankstelle and other classics of German cinema, sadly also including the Nazi big-budget production Jud Süß.
In the group photograph, we can see clearly the striking geometric designs that he created for part 2 of Die Nibelungen. They employ a mix of concentric designs, diamond shapes, and figure-ground reversals.
To my unschooled eye, they appear to be severely simplified idealizations of Gothic or Gothic Revival Style.
Neo-gothic murals in Agen cathedral (12th century). Image credit: by Danielclauzier (Self-photographed) [CC-BY-SA-3.0 (http://creativecommons.org/licenses/by-sa/3.0)], via Wikimedia Commons
Hunte was a graphic artist with a consummate knowledge of light and shadow. A collection of 68 of his set drawings is shown on European Film Gateway, with about two dozen of them from Die Nibelungen. While none of them feature the striking wall patterns of the movie sets, I believe it is a safe assumption that Hunte both drew from historical medieval ornamental art and at the same time took care to maximize contrast so that they would come out as vivid as possible on black-and-white film stock.
The costume designer is usually credited as Paul Gerd Guderian. However, according to this weblog by British film maker David Cairns,
... Aenne Willkomm, brought in to design the Huns’ wardrobe, had been
working in high fashion. She was recruited to help designer Paul Gerd
Guderian, who was suffering from tuberculosis. Guderian actually died
during production, leaving her to finish the film.
A book by someone named McGilligan is cited as the source. I would disagree with Cairns, however, that
... the design of Die Nibelungen's sets and costumes [eschews]
historical accuracy ...
While clearly everything in the film was done with an eye to how it would come across on the silver screen and historical accuracy was not the paramount goal, nonetheless the influence of medieval ornament and pattern on the costumes, too, is evident:
Double page of the Reiner Musterbuch (Rein pattern book), early 13th Century. Image credit: Public domain, photograph taken by Henryart, via Wikimedia Commons
Compare these patterns to the photograph shown on Cairns' weblog. Whether one calls this "inspired by" or "based on" is secondary, I think. But of course, other influences such as Neue Sachlichkeit and the constructivism of artists such as El Lissitzky also played a part. (Compare Hunte's scene drawings to the art of El Lissitzky on the previous link.) The movie is sometimes mistakenly labeled "expressionist", but the expressionist era had already passed well into history by the time of its creation.
Die Nibelungen was a huge project for the Ufa production company. Entire forests with thirty-foot trees were installed on the set. A seventy-foot dragon with humans inside to move its limbs was built. Lang and his staff spent three months just meeting to discuss creative decisions and technical planning for the film. If something was big, it was too small. Lang would not settle for anything short of monumental.
Recently a restored and remastered copy of the film, based in part on recovered original camera stock, was released. It preserves the black-and-orange virage coloration and reintroduces some of the footage cut by the producers from part 2. A bloody saga of honor and loyalty, vengeance and downfall, which some critics have said prefigures National Socialism. Lang, however, hightailed it out of Germany as soon as Hitler had taken the reins of power. In a letter to film critic Lotte Eisner, he even hinted that the Huns who slaughter the decadent Burgundians may be an allegory for the proletariat overthrowing its oppressors.
Parts of the film, with its human-operated dragon and obviously artificial scenery, may appear ludicrous to modern audiences weaned on Hollywood CGI productions. But as a historical document, it is without parallel.