According to Martin Harris, Joseph could not translate the BOM without the gift of God.
Martin Harris, letter to Mr. Emerson, January 1871, in Saints’ Herald 22, no. 20 (October 15, 1875): 630, M291.5 S157 v. 1- 1860-, Church History Library
I do say that the angel did show to me the plates containing the Book of Mormon. Further, the translation that I carried to Prof. Anthon was copied from these same plates; also, that the Professor did testify to it being a correct translation. I do firmly believe and do know that Joseph Smith was a prophet of God; for without I know he [Joseph Smith] could not had that gift; neither could he have translated the same.