When I lived in Russia, a friend of mine spoke Russian sign language and I asked how they managed to communicate in the cold winter months outdoors. They told me using a lot of facial expressions, body language, and that you can sign a surprising amount even wearing gloves.
Mittens apparently are the equivalent of gagging somebody though.
A list of the ones that are being used in different regions/countries:
* https://en.wikipedia.org/wiki/List_of_sign_languages
* https://en.wikipedia.org/wiki/List_of_sign_languages_by_numb...
Also, for the very young:
* https://en.wikipedia.org/wiki/Baby_sign_language
Wonder if it'd be easier to create a 'universal' sign language than a universal spoken one (e.g., Esperanto, or an old lingua franca like Latin).
This is a prime example of isolationism's contribution toward structured communication needed to function adequately in an agrarian society.
There's been a lot written about MVSL. Oliver Sacks' book "Seeing Voices" is a good introduction, it has a chapter or maybe a whole section about it.