Question about "bon appetite"

How exactly is this phrase used?

Is it same as Japanese "itadakimasu" that is said at the begging of meal,

or is it like serbian "Prijatno" that you might say when you walk in on someone eating, expecting him to say "merci".

I'm wondering this, cause if serbs catch you eating, and tell you: "Prijatno(=Bon Appetite)" you are expected to reply with "Hvala, izvol(i)te (=Thank you, please (join me))" which I find to be quite a mouthful when your mouth is full, and it unfortunatelly goes as far as for the person to stand in a doorway until you swallow your bite and respond, which is extra annoying. One coworker even asked me "did you not hear me" after I failed to respond at alloted time, even though I ranted about that custom for an hour day before.

So, please?