Evidence suggests that Joseph Smith's translation of the Book of Mormon was a "tight" translation - i.e. one in which the words were provided to him by means of either the Urim and Thummim or the seer stone, and Joseph himself had little or no input into the words that were used.
From Richard Bushman’s "Rough Stone Rolling," page 72:
"Close scrutiny of the manuscript (by a believing scholar) seems to support transcription. Judging from the way Cowdery wrote down the words, Joseph saw twenty to thirty words at a time, dictated them, and then waited for the next twenty to appear. Difficult names (Zenoch, Amalickiah) were spelled out."
This would suggest that Joseph was reading the translation rather than creating it.
Royal Skousen's Book of Mormon Critical Text Project
makes the case that the book was written in Early Modern English, an idiom that was common to the 13th Century, not the 17th Century King James translators of the 19th Century Joseph Smith. This would also strongly suggest a tight translation.