When French-speaking photographers want people to smile, they might say Le petit oiseau va sortir (The little bird is going to come out) or Souriez! (smile), or might ask them to say pepsi! or ouistiti! (marmoset), just as English-speaking photographer get people to smile by asking them to say “Cheese!”
The word ouistiti [ˈwistiti] means marmoset in French, and is apparently imitative of the animal’s cry.
Another French word for marmoset is callitriche, which comes from callithrix, a genus of monkeys found in South America that includes some species of marmoset, and which comes from the Greek kallos (beautiful) and thrix (hair). The callithrix are part of the Callitrichidae family, which includes all marmosets and tamarins found in South America. The marmoset in the photo above is a Pygmy marmoset, or Cebuella pygmaea.
The word marmoset comes from the Middle French marmouset (gargoyle; small child), which probably comes from marmouser (to mumble) [source].
Other equivalents of “Say cheese!” can be found on: http://www.omniglot.com/language/phrases/saycheese.htm – additions and corrections are welcome (as always).
What do you say when you want people to smile?