bon appetit! [expression]
This is a command form and it translates to "eat tastily" or "eat with taste".
Some of you might feel that we really need this wish in the Netherlands! 
An equivalent expression is: "Eet smakelijk!".
Related expression:
When you leave the table and others are still eating, you can say: "prettige voortzetting!". This means: "pleasant continuation".
Extra:
1. Read more on Dutch food and eating habits:
www.thehollandring.com/food.shtml
2. Go Dutch on some recipes!
www.godutch.com/windmill/recipes.asp?id=all&hist=2
two teenagers sat down at the next table and said “eet smakelijk” to me at the Amsterdam McDonald’s, which i thought was cute – and kind of funny. Smakelijk? McDonald’s?