It's a Colourful Language Out of Space
Whilst exploring some ruins, looking for traces of an expedition that went missing some months ago, we encounter an ageing witch doctor tailing us. We confront him, and he turns out to be mostly friendly. He tells us that the expedition had indeed come this way, and left. That's a starting point.
After buying a charm from him, the witch doctor leans in to me and whispers something in my ear. The language sounds almost but not quite like Spanish, and having said it, the witch doctor runs off in to the forest.
'How peculiar. Professor, you're knowledgable of the local dialects, I think he said something like, "Iä, Iä, Cthulhu Fthagn". Whatever could it mean?