While it’s known that Wookiees can understand Galactic Basic (the equivalent of English in Star Wars), they do not appear physically able to speak languages that fall outside ones known on their home planet of Kashyyyk.
Jon Favreau and Dave Filoni may have worked that Jawa diss in there just to make a funny, though it’s worth noting the joke touched on an unanswered bit of Star Wars lore.
Basically, this Jawa burn may have cut a bit deeper than it seemed, which may be why Din shot his flamethrower at the little scavengers.