@ mstr kenobi: Actually, you'll notice that most of the people who talk directly to Wookiees are Jedi. Also, Han has been around Chewie for a long time so he gets what he's saying since he understands the language. Same with Lando as well since the three were friends for a long time before the movies.
Other times, like with Leia, she gives Chewie orders and he acknowledges but she never really has a conversation with him since she's not as adept with the Force to communicate as well with Chewie as Yoda, Obi-wan and Luke seem to, or have the experience with his language to understand him as Han does.
Also, as I said before, Wookiees are technologically advanced and have created their own technology. Their form of communication is definitely more than just simple verbal communication. In all likelihood, it's a language that consists of great subtlety and other factors of non-verbal communication that come into play as well.