Since it's an international flight, your parents will have to go through customs before you can meet them. I don't know of any airport that allows people to meet/greet arriving intl pax before they pass through customs, it's just a safety thing.
I would have to assume IAH has people that can assist in these types of situations. When they get to customs, someone that speaks their native language will be brought in to complete their arrival
Also, if they plan to travel back here in the future, I would try to learn at least a couple of key phrases. TSA/CBP employees can be frustrating at times, it helps to guide them in the right direction right off the bat.