In my opinion, Jerry would still have felt the need to prove himself. In fact, he might even have felt more pressure to do it.
In the story, the boys make Jerry feel accepted. They do not actually do anything to put pressure on him to go through the tunnel. He is the one who feels that he is a failure when they can and he can't.
But imagine if they spoke his language. I think that they might start to make fun of him. They might actually come out and tell him that he is some little baby (and a foreign baby at that) and that he should run off and find his mommy.
In the story, Jerry puts all the pressure on himself. If the boys could have spoken English, they might have added to the pressure by taunting him.