Oliver Cowdery says Joseph used the Nephite Interpreters to translate the BOM.
Oliver Cowdery, Letter to W. W. Phelps, in Latter-day Saints' Messenger and Advocate 1, no. 1, (October 1, 1834): 14
These were days never to be forgotten— to sit under the sound of a voice dictated by the inspiration of heaven, awakened the utmost gratitude of this bosom! Day after day I continued, uninterrupted, to write from his mouth, as he translated, with the Urim and Thummim, or, as the Nephites whould have said, “Interpreters,” the history, or record, called "The book of Mormon."