as a native english speaker, i have never heard anyone say "good appetite." Bon appetit sometimes. Maybe a better translation would be that (despite it being a french phrase technically.) Or "eat up!."
When you use an expression like “Good apetite!” in English you are actually shortening a full statement.
Bonan apetiton! = Mi deziras al vi bonan apetiton! (I wish you a good apetite!)
I've never heard anyone say "good appetite" in English. We always say "bon apetit" (French), or just say something else that conveys the same message (Enjoy your meal, etc.).