They should come up with 1 universal sign language system and make everyone in every country on planet earth learn it in school. Wouldn't that be the easiest way for everyone on the planet to communicate with each other ?
This would be quite ideal; however, like all languages, things change and evolve over time. Especially if two groups of people don't communicate with each other for several years.
For example, French, Spanish and Italian used to be one language, but people migrate, groups form, and people break away from the tribe/ community. Eventually, the languages that used to be one split and become different from one another.