Wish You Good Appetite
In Germany or France people usually say something like: wish you good appetite before starting a meal like dinner. In English there doesn't seem to be something like that. Apart from bon apetit said by some people who belong to a higher class there is: enjoy your meal usually said by waiters. Peopel might start, when in company, just by simply nodding signalizing you can dig in now? The word apetite is of French origin. This means there seems to be no Germanic equivalent to Romance apetit.

Other possible more indirect ways are: Let's eat; help yourself Please, do start, please don't wait for us - do start; don't let your meal get cold; please start. Also 'Dig in' [informal], 'Tuck in' [less so, but still not formal], or 'Serve yourself'; usually, even if plates are delivered ready-filled from the kitchen, there's at least one thing that guests can help themselves to: 'Help yourselves to vegetables/salad/gravy...'- whatever.

In very formal circumstances, a Grace is said; some private households still do this for religious reasons, and even in not-very-religious ones (with a sense of owing something to someone or some principle) I've heard 'Blessings on the meal'. The standard Grace before Meals (it might even be in the Book of Common Prayer. The shortest I've heard is the Latin 'Benedictus benedicat' [may He who is Blessed, bless]. (This isn't to say that everyone in the UK is automatically prayerful; but ceremonial behaviour of various kinds often has the trappings of religion.)

No one should begin eating until everyone has been served. No "signal" per se is necessary; once everyone at the table has been served, then dining may begin. If you're at a dinner party, the host is traditionally the last to be served, so once he or she has his or her food, the rest of the table may begin eating.

Students of English are often confused when they learn there is no polite English word for wishing good appetite. They wonder why on the one hand the British have a long-winded manner when it comes to politeness: if you don't mind, If I may say so, would be so kind as to..., do you think.. and thousands of: sorry, excuse me...... but on the other hand there is no polite way for wishing good appetite. Isn't etiquette a little bit like grammar? Do you have to follow the rules because they're written in golden letters, or is common sense more powerful?