If a deaf person from China, Russia, Brazil and America come together and they all knew sign language but do not know each other's language, can they communicate with each other? Or if a deaf person is travelling in another country where he doesnt understand the (written) language and gets hopelessly lost, can another deaf person or sign language practicioner who otherwise doesnt speak English or the lost person's language, be able to communicate with them?

In languages when it comes to learning a language out of all the languages sign language is one of the most simple languages to learn and I think it's easy because when you look at sign language the signs are easy to memorize and if you have a sign language dictionary handy you can master at least some of the signs overnight or in a week. As for sign language in other languages I believe there is French Sign Language because how American Sign Language was invented the guy who invented it met a deaf guy in France named Laurent Clerc who used French Sign Language and he actually taught English signs to Laurent and American Sign Language was born. But I have never ever seen Spanish Sign Language or Italian Sign Language before. When I was in school I used to love to see my teachers use sign language because I thought it was amazing and I do know some signs myself
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https://en.wikipedia.org/wiki/Sign_language#Relationships_with_spoken_languages