the way that Stephano's reaction to Caliban weakens the power that comes from speaking the same language would be: Stephano is intrigued that Caliban knows the English language yet still views him as a savage animal.
Stefano refers to Caliban as a 'brave monster' because of his wild appearance. At first, Stefano thinks Caliban is some sort stupid islander, and he became surprised after hearing Caliban communicate a point with him.