"תודה רבה, בתיאבון."
Translation:Thank you very much, bon appetit.
Good point. But I think of בתאבון as what the diners say to each other. In my family we said בתאבון or bon appetit when we sat down to dinner. You wouldn't say "enjoy your meal" at a family dinner. The accepted translation is the French, even though it's not common. The bottom line is that it's just not part of the culture for most English speakers.
On a side note, I see "family dinners" as rapidly becoming a thing of the past. My family used to have Friday night dinners together, until my grandparents' generation passed away. Now, we have dinners with clients and friends, but not family. They're all off doing their own thing. As for what we say at the beginning of the meal? "Enjoy!"
To put to rest the comments. Yes we use bon appetit in America. American English has a few French words and phrases due to the fact that France helped us in the revolutionary War. We still say a couple of things such as bon appetit and adieu. They are not super common anymore but they are now American English words according to the dictionary.