Brant A. Gardner addresses the use of "adieu" in Jacob 7:27 notes that it is a valid English translation of whatever word that was on the plates Joseph translated.
Brant A. Gardner, Second Witness: Analytical and Contextual Commentary on the Book of Mormon, 6 vols. (Salt Lake City: Greg Kofford Books, 2007), 2:579
Translation: It might seem unusual that the French word adieu is in a text that purports to be a translation of a document written prior to the development of the French language. However, while it is certainly true that French should not be on the plates, it is equally true that English should not be on the plates. It is no more surprising to see a French word in the translation than it is to see the English words in the translation. Adieu literally means “to God,” and was part of the basic vocabulary of English speakers during Joseph Smith’s time (and remains part of English today). The term adequately communicates the meaning intended.