What is the best assistance or help that I can arrange for elderly parents who don't speak English?

I'll assume that you're worried about the flight and how they will get through the passport control and customs in the USA...

Most airlines have staff that are able to speak multiple languages. Most staff will speak at least two. If the airline is flying to where your parents live, it's more than likely that there will be staff on board that speak their language.

You should request assistance from the airline - they will be able to help your elderly travellers through the passport control and customs on the USA side. In addition, as @AdityaSomani said, prepare for them a list of common phrases with pronunciation written in their native language. Finally, write a letter from your own name addressed to the border agents, detailing their relationship to you and the purpose of the visit - and include your contact details.

I live in England and when my mother-in-law visited us for the first time, this is exactly what I did. She didn't speak any English at all. We wrote the letter as I described and requested assistance from the airline, who were happy to help. They assisted her through the passport control, where she handed the letter to the agent, and then got somebody from the ground side to help her with the luggage.