The thought that Jason Momoa didn’t speak English is understandable, considering most of his lines inGame of Thrones are in Dothraki.
That said, Dothraki is a fictional language, so unless the people who watched Game of Thrones thought he was just speaking an obscure and unfamiliar language, it’s hard to understand why they just assumed he didn’t know how to speak English.
Additionally, though Momoa wasn’t as big of a star prior to Game of Thrones, he had enough sizable roles in shows like Stargate: Atlantis and Baywatch that producers could’ve easily figured out he spoke English had they looked into it a bit deeper.