There are more than 300 sign languages in use in the world today. When signers of different languages meet, how do they communicate?
Sign languages operate like any other language, in that they are rarely planned in advance and grow organically as they change and develop. And, like spoken languages, there are a lot of them. Sometimes their territory overlaps with spoken languages or specific countries, but sometimes it does not. Case in point: American and British Sign Languages are mostly mutually unintelligible; American Sign Language is much closer to French Sign Language. See the video below for more detail.
Given this wide diversity of sign languages, how do signers of different languages communicate with each other? Well, when two signers get together they have to find common ground – both the words that they use (vocabulary) and the way that they construct sentences (grammar).
For the former, well, there are some shortcuts. International Sign (IS) is an attempt to provide a baseline set of signs for everyone. It’s not codified and written down, but is more like a pidgin language: an impromptu attempt to find enough commonalities to be able to communicate using the toolsets of both signers’ languages. As you can imagine, you have to simplify everything and rely a lot on pointing, mime, and fingerspelling. But it works.
(Side note: the World Federation of the Deaf they did try to formalise IS in the 1970s. That attempt, called Gestuno, didn’t really take – like most planned international languages. Esperanto, Volapük, Interlingua…)