Hi,
Do you know how to say “bon appétit” in English? It doesn’t exist. We usually say “Good luck”!
Can I say “Do you know how they say …”?
Thanks. Bye bye
To be precise, your sentence would have to be, ‘do you know how they say ‘bon appetit’ in English…’
By the way, the sentence is untrue.
Bon appetit = enjoy your meal
Bonne chance = good luck
Bev, I quoted what I found in a book.
Thanks a lot. 
Hmm… let me understand. What does that “to be precise” mean, exactly? Is “how to say “bon appétit”” uncorrect, or just less correct?
To be precise = to be accurate.
Your version is grammatically incorrect.
Ciao Bev, thanks a lot.
Apparently books (for learners) can contain a few errors, sometimes.
To clarify:
‘how to say…’ (the original version), is grammatically correct.
‘how they say…’ (your version), is grammatically incorrect unless you change ‘English’ to ‘England’.
Although the grammar is correct, the statement made in the original sentence is incorrect.
So you wrote “English”, while you were thinking “England”. 
Thanks again, Bev.
Yes, sorry. I didn’t notice that until you just pointed it out and I scrolled up.
We got there in the end.
To use ‘how they say’ you would have to use the name of the country (which relates to the people indicated by the pronoun ‘they’) rather than the name of the language.
Of course, it doesn’t have to be ‘England’ because English is spoken in other countries too.
Now it’s definitively clear.
You are the perfect teacher!
As in “Good luck eating that”? :-)))
Exactly. This is the sense of that “Good luck”.