The Book of Mormon quotes from the 1769 version of the King James Bible, including using italicized words that are unique to that version. That strongly suggests that Joseph Smith simply plagiarized the Bible to write his Isaiah sections.
This question is, of course, a contextual consideration of how and when 19th Century words make sense in view of the claim of authentic translation. Also, the question itself, rests and is most properly considered within the greater context of a variety of related questions that also arise on the subject of language used in the professed "translation" and other similar contextual considerations.
More on those contextual considerations here.
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