Although Juana's hair is made of gold, the bishop can cut it easily with hairdressing scissors, while later the mayor is seen cutting the same golden hair with pliers.
Juana is seen with a different body shape between scenes,that's because of the long time it took the shooting of the film.
The bishop makes a chemical test to confirm if the hair of Juana is real gold or not. Indeed, these tests are performed with certain acids and chemicals that react with these metals, but this test can not be performed as shown in the film. Gold or other precious metals can not be tested into small, thin fibers (such as the thickness of a hair), but in ounces weights or higher.
Bishop shows to the people that the Juana's hair is truly golden hair, but in the shot where we see the hair in a container with liquid, the hair is floating. If were golden hair, it would be at the bottom of the container, not in the surface, since Gold is a metal.