If you mean his spelling, as in spelling errors, I've something to add. Tolkien was fiercely protective of what he wrote, including his errors. When he found something miswritten in his manuscript, he was more likely to ponder, in terms of Middle-earth, how his characters came to make such an error, or what special significance this might have, than simply to correct it. Thus, a mispelt foreign word was more likely to remain as an example of regional dialect than to be changed. I don't believe there is a single error in any of his writings that does not have an explanation of sorts.