What I would focus on is
what is going on with your fiction? What's it for? As in, what's its point?
Eg if your fiction posits an essentially human people who live adjacent to, and in contact with, pseudo-High Mediaeval France but have technology comparable to Australia at the same period, what is going on? What are you trying to achieve or communicate?
Describing these people as
primitive seems to reinforce the sort of essentialist idea that
@Aldarc described upthread. I just don't see what it adds.
Eg if you want to say that a particular language is not normally written down, I think maybe you just say that. Again, though, the question might be
why? What are we meant to take away from this fiction? I can imagine a culture that, for some reason - in the context of a FRPG, perhaps reinforced by supernatural imperatives - rejects the written word. But then we are not talking about "primitives" - we're talking about traditions or taboos.
And of course the Cormyr-type peoples probably have traditions or taboos too - presenting them as normatively typical, while presenting the non-literate culture as "primitive" or deviant, seems like a way to reinforce racialised ideas.
I don't know if this is quite the answer you were looking for, but I hope it is a contribution to the conversation.