The first time I read that in Jacob 7, probably as a 12-year-old boy, I got excited to realize that Jacob spoke French! I can vaguely remember my dad taking a stab at explaining to me the reason, but it didn't make sense then. I get it now.
Joseph saw everything in the plates through the lens of the early 19th century. He had to use what he knew when he made sense of the words that came to his mind. So when he translated a farewell phrase that Jacob would have used, he translated it to the 19th century equivalent: "Adieu"
It's fortunate that he didn't know the phrase "Peace Out".
I know some people get hung up on how similar the sermons in the Book of Mormon are to early 19th century sermons, but how could they not? That's the language Joseph spoke, and the language into which he translated the plates. It's entirely possible that what they actually were saying was slightly different, maybe even significantly different. The point is, there actually were men saying something, and even if he didn't translate their colloquialisms exactly, he got the doctrine.
