Many people are probably wondering why there was a caveman in the episode "The Time Capsule". Hopefully I can give a good enough theory for the appearance of the caveman.
Where as most people believe Thundercats is in the future, the explaination of the caveman can be easy. The caveman lives in the cave because his home was destroyed and everything along with it. He was forced to find refuge in a cave.
Now for the reason why the caveman wouldn't/couldn't talk but could understand English can also be easily explained, I think. Where he's human, he can understand English.
A possible reason why he wouldn't speak could be, he's still in shock from losing his home or the person who destroyed his home, could have his wife and children and he's not allowed to speak to anyone if he ever wants to see his wife again.