Ask any fans of Japanese anime or films how to say “bon appétit” in Japanese and they will immediately respond “itadakimass”, even if they don’t speak the language. In reality it is a bit more complicated than that.
This “translation” is actually a shortcut made possible because, in both cases, this is something we say before a meal. However, 頂きます itadakimasu does not exactly mean enjoy your food, but rather "I receive" or even by extension “I ingest” (food and drink).
It's an expression of humility which expresses gratitude. So itadakimasu is not a social nicety but something rather more spiritual. You don’t say it to a group and cannot wish it to others, unlike the expression “bon appétit”, but it is something you say to yourself. So you can say it even if you eat alone.
It is said not only to the cook but also to those who have put the food on the table, even down to the individual ingredients... The host or the chef, if any, might also say 召上がれ meshiagare (“to get excited”) indicating that the meal is served.
One can make a slight bow of the head while saying itadakimasu, clasping the palms vertically while holding the chopsticks horizontally between both thumbs.
And there is no need to reply “thank you” (arigato) as this would make no sense.
During and after the meal
It is considered good form to enthuse with 美味しい oishii! (“it's good!”) and also 甘い umai! (“it's delicious!”) after the first bite, as if you had never eaten anything so good.
At the end of the meal, there is the same process of politeness as at the start, ご馳走様(でした) gochiso-sama (deshita) which literally means “it was a feast”.