Why is the word “adieu”, a French word, (French is a language derived from Celtic and Latin around 600 A.D.) uttered in Jacob?
The French word “adieu” was never uttered by Jacob. Jacob didn’t speak French. He spoke Hebrew, and was acquainted with Egyptian writing. As you mention, the French language did not start to be developed until after 600 B.C. The Plates from which the Book of Mormon was translated were written in Egyptian hieroglyphs, which is not even a phonetic language.
In a pictographic language an idea is expressed in non-phonetic terms. The phonetic interpretation of the pictogram is entirely the product of the translator. The translator has the obligation of conveying the sense or meaning of the pictographic depiction. The choice of “adieu” rather then goodby, adios, ciao, so long, or any other parting expression is entirely at the choice of the translator as long as it conveys the intended meaning.