There’s no much of a sign language. At least most drivers know of any.
A little bit that does exist:
Raising you arm = I want a ride. The details vary and don’t carry any special meaning. A raised thumb is not common, but not rude either (might be interpreted as thumbs-up, which is weird).
A common alternative is holding a sign with your destination.
Driver pointing downwards = his destination is very near. He would stop, but you probably don’t want to take a ride with him.