I have a question about a title.
They say they can see souls. People give them information about the dead, and their job is to draw a map where the dead's body lies.
So what are they called?
I think they are real because they are able to find bodies. There are a lot of youtube videos about them (unfortunately in Vietnamese).
If I translate their title into English, what should I say?
Does 'spiritist' work?
I don't think we have a name for this exact thing; as I said, I've never heard of it before. A "psychic," as suggested by Liliana, claims to read people's minds. A "medium" claims to speak to the dead; that's close, and you could use it with a little explanation. A "spiritualist" is someone who believes generally in communication with the spirit world; that might work, although it's a little generic. I suppose you could create your own name: "soul seer" works about as well as anything I might come up with.
As the saying goes, a broken clock is right twice a day.
The people who do this probably do not call themselves one thing because they (claim to) have different methods as pob14 mentions. Someone who knows where the body is by talking to the ghost of the dead person might call themselves a medium, a channeller, or various things along that line. If they know where the body is by reading the mind of the killer, they might call themselves a psychic, a mindreader, ...
I wouldn't use "seer" to mean "one who sees" in this context, as people would assume that you meant "someone who predicts the future".
I've occasionally seen a US TV show about using 'psychics' to help in cases of missing or kidnapped people. 'Psychics' who specialise in this sort of work could be called 'psychic detectives'. Or maybe they call themselves this, I am not sure. I have never heard the phrase 'soul-seer' and wouldn't know what it meant. It's possible that it is a familiar phrase to people who believe in the paranormal. Psychics either believe in their powers, or they are deliberate frauds. Either way, they exist.