When I was learning sign language, I asked a Deaf person "what do you say when someone sneezes?" He looked at me askance. "I mean, what do you sign when someone sneezes?" His look was equally confused. "Nothing." He signed. "We don't do anything when someone sneezes, unless it's gross and they need a tissue."
When I was learning German, it was easy and automatic to say "Gesundheit" when someone sneezed. It was actually a bit embarrassing. One day, some one sneezed, I said "Gesundheit" and then "Oooooooh!" as I often did when using a word like that or "Kindergarten" or "schadenfreude" or any other word that I had taken ownership of through my learning and collecting of the german language.
In fact, I said gesundheit so naturally that it put off a fellow teacher in the kindergarten where I worked. She and I were the English teachers and, me being the native speaker, she deferred to me often in questions of more casual speech. "Jessica, we're meant to only speak english in front of the children." Of this, I was aware. Absolutely none of the children at school knew that this teacher is actually Swiss and simply speaks flawless English without an accent. "I know. That's what I say in the states, too." This teacher said "God bless you", something that I had not said in years and years.
In class, my french teacher was terribly polite and impressed upon us the importance of certain niceties while we are here in Paris. Answering and initiating pleasantries and the inclusion of Madame or Monsieur whenever possible, and so on. She taught us that someone sneezes we say "A tes sohuait." I asked what we say in the polite form and she told me not to worry about it. I pressed her again and she seemed please and told me.
The other day, when in a sparsely populated metro I was sitting next to a stressed-looking man, when he sneezed. "A ves Souhait", I said quietly while wearing my ipod. He was so pleased that he lightly tapped the inside of my elbow while saying thank you.
I liked that teacher.