Nope, sign language is not universal. Like any spoken language, it just takes two or more to talk and consequently develop into a mutually agreed code.
I've heard of a family in the countryside in which all the children are deaf. They've developed their own sign language that makes no sense to anyone outside of the family.
I saw three men signing in Nebaj, but they didn't understand my signs for "deaf" and "name". Two were hearing, so I guess they developed a code just among the three of them.
We found that signs in Guatemala City are different from signs in Xela (Quetzaltenango), but since we're working in schools in this region, we'll base the signs locally.
Even with finger spelling (ABC), I have to switch between Lao, Ghana, ASL and now Guatemala systems. But you'd be surprised how it's not that hard once you get the fingers moving.