A: No more so than there is one spoken language for all countries! There are hundreds of known sign languages, with regional dialects, and new national sign languages are being created (as is happening in Nicaragua). American Sign Language is used throughout the United States and Canada (which also has LSQ, the sign language of Québec). Although ASL possesses numerous regional variants, a signer from California can easily understand one from New England.
Similar signs can have vastly different meanings in different sign languages. What is a perfectly polite sign in one language may be an obscenity in another.
International Sign Language” is used at international conferences. “Gestuno,” a signed version of Esperanto, was developed in the mid-1970s by a World Federation of the Deaf committee, so it’s really not a language; the term “Gestuno” has fallen out of favor. It’s a very simple, logical, universal system of signs, used to convey greetings and instructions, that is immediately understood by all signers, whatever their native language.
Participants at the Deaflympics and other international events who use different sign languages quickly find ways to establish communication with each other. They “invent” their own “international sign language” as they go along.