We clearly see that characters talking to Wookies in Common (English) or whatever and Wookies understand what they're saying. We also see that Wookies just respond with their "Rawrrrghhhs" and that usually the receiver of the response understands them back. I am aware that there are a few Wookie languages, but we hardly ever see any non-Wookies reply back in any of those languages. So what I'm suggesting is...is it possible that Wookies are telepathic in some way? That they can control what message is to be conveyed to the receiving party by their mind, though vocally it just all sounds like "Rawrrrghhhh!"