Seems every culture/nation wants / has  developed its own 'dialect'

I believe there is an international sign language, at least in intentions.   Can a deaf person that only knows a particular variation of sign language from his country, understand a 'decent' amount of international signs to carry a cross conversation ?

If such international language was someday a reality; could it bypass spoken languages as a -say chinese- sustain a sign conversation with a -say hebrew- ?  And all other cultures  permutations, without learning all the involved spoken languages themselves ?

 

They are not different dialects, they are totally different languages. For example British Sign Language and American Sig Language are completely different and not mutually comprehensible. (And note that there is no connection between the spoken languages in a country and the sign languages.)

There is an International Sign Language but it is a "pidgin" with limited vocabulary and grammar. It is used when people who do not share a common (sign) language meet. https://en.wikipedia.org/wiki/International_Sign

An true international sign language is probably not practical as it would require everyone to learn a second language. Look at how successful things like Esperanto or Volapuk have been.

Wow! Never thought about it