This is driving me nuts, so I’ll say it again okay?
"Je Suis Prest" is old French derived from Latin. Mottos don’t change over time. So no it’s not “Je suis prêt” as we would normally say in French today, it’s “Je suis prest” the latin derived motto.
If you ever come to Quebec City you’ll see loads of places that have old French inscribed on them. The City’s motto is “Don de Dieu feray valoir” which is old French. We don’t speak old French anymore, but the motto remains the same. Because that’s how it works!
One last time… Mottos and crests do not change over time. Also Outlander takes places in the 1740s a period during which old French was still used. Does this make sense to everyone?