The challenge of appreciating art from another culture is that we don’t know the codes, understand the backstories, or share the references that made it so resonant with the original audience. The barrier is often lowered with Old Masters paintings because of the familiar classical and religious references. But when it comes to portraiture, especially from 18th century Georgian England, ordinary or seemingly straightforward images can turn out to be something else entirely. That’s what’s so interesting and refreshing about the Frick Collection’s new show, Gainsborough: The Fashion of Portraiture, which opens Thursday. It may not fully decode the complex social world of Britain’s “first fashion,” but it does give viewers the tools to relate to images that would otherwise seem stuffy and remote.