Chewie most likely understands galactic basic (English), as why he understands Han when he speaks. Han on the other hand probably understands Shyriiwook. (The language of wookies) He could have learned this in tons of ways so I won't go into that. This wouldn't be the first time something like this has happened, an example of this would be the protagonist of KOTOR and Zallbar the wookie who could both understand each other through the protagonists understanding of Shyriiwook and Zallbars understanding of Galactic basic. The protagonist of KOTOR however could speak Shyriiwook and many other alien languages.