English speakers don't have a saying like "Guten Appetit!" which they say to each other before starting to eat. If you would like to say something, "Enjoy!" or the phrase "Enjoy your meal!" are the best options. Some English speakers might also use the French phrase "bon appetit". Don't use the less formal "Dig in!" (Hau rein!) or "Let's eat!". If the meal begins with a toast, it's common to say "Cheers!"