There is a US Airforce custom of saying, "Howdy!" after farting in flight, to assure one's colleagues that one's intentions are peaceful.
Jimmy K. tells of a friend who says, "Speak to me, oh, toothless one!" Similarly, Bazzbull says, "Speak to me, ol' toothless wonder!" And EHALL says, "Oh, toothless one! Your voice has changed but your breath is still the same!"
TGeaber says, "It's that asshole talking behind my back again!"
Gpgo and his frat brothers say, "Oops! A sudden loss in cabin pressure!"
Murray E. sends this account of a response to belching: "Once, when Dorothy Parker was at a dinner, a man burped quite loudly she is reported to have looked at him and said, 'why didn't you use the other end and save your teeth?'"
After Czechs let out a silent fart, they announce the fact by saying, "Soytka," to let their friends know they might be smelling something. The word refers to a kind of little bird.
Rodney's father says after farting, "Speak up, Asshole - Don't take that shit!"
Another father, that of LeNaye, says the following: "There's a kiss for you!" or "The ducks are calling." or "Damn frogs..."
When someone else farts, Mack says, "Oh, stop your bragging!"
The thing to say in northern England after farting is, "More tea, Vicar?"
A Royal Navyman, would say, "Don't worry - plenty for all!"
Bill J. and Matt K. say, "When farts are transported past another while still engulfed in one's pants, one should ask the perpetrator, 'Are you draggin' skags??'"
Rodney Dangerfield says, "Hey, did somebody step on a duck?" in Caddyshack.
In Mexico, what you say is, "Reza por tu alma porque tu cuerpo ya esta podrido." This means, "Pray for your soul because your body is already rotten."