I'll only address the question from the translation requirement POV: what would you call a person who uses fetishes for magic, given the culture is African (as opposed to, say, Haitian), but in the context of Western literature?
Given various factors, I feel this would be more appropriate than actually giving a nuanced description of such a person in African culture:
- the age of the book, when Western fiction would often have stereotypical portrayals of African culture
- various lines in the book itself ("I was but newly come to the East Coast, new to African ways [...]. Because I came from Virginia, race instinct and prejudice were strong in me [...].") suggesting that the book itself does not intend to portray African culture accurately
Let's first look up fetish. From Merriam-Webster:
- a : an object (such as a small stone carving of an animal) believed to have magical power to protect or aid its owner; broadly
: a material object regarded with superstitious or extravagant trust
or reverence
A fetish in this context is like a voodoo doll in that it lets someone access magic. Unlike voodoo dolls, fetishes don't have to be doll-like. They can be plain objects or even parts of bodies (human or animal). Correspondingly, a fetish man is like a voodoo priest in that they access magic using an external object, often with an accompanying ritual.
For translation, I'd suggest that you look up some translations of Tarzan novels. There are a few witch doctors in them, and some of those witch doctors use fetishes. Since the these books are pretty popular and mostly public domain in some countries, you might find good quality translations already existing in your language, which you can then rely on for familiarity for the readers. For example, from Tarzan and the Leopard Men (particularly appropriate since it was published in the same era, and also has a hyena fetish):
Brandishing his hyena tail, he leaped toward Muzimo. "Die!" he
screamed. "Nothing can save you now. Before the moon has risen the
third time you will be dead. My fetish has spoken!" He waved the hyena
tail in the face of Muzimo.
The white man stood with folded arms, a sneer upon his lips. "I am
Muzimo," he said; "I am the spirit of the ancestor of Orando. Sobito
is only a man; his fetish is only the tail of Dango." As he ceased
speaking his hand shot out and snatched the fetish from the grasp of
the witch-doctor. "Thus does Muzimo with the fetish of Sobito!" he
cried, tossing the tail into the fire to the consternation of the
astonished villagers.
(The books are horribly racist and misrepresentative of African culture, but still likely to be familiar with fans of science fiction or fantasy.)
As a personal opinion, the exact name used for someone who uses fetishes for magic is not very important. Burroughs uses the term witch doctor, or even shaman. Usually, I have seen fetish magic applied in fiction by magicians in a tribal context. For example, with terms like shouldermen (guess the fetish!) or bonecaster in the Malazan Book of the Fallen, the meaning is apparent if you have seen such tribal magic elsewhere in fiction. I think you could use whichever of these terms or their translations is most likely to be familiar with your target audience without much loss of fidelity.
