WE DON’T THINK OF the Russians as being “casual” about anything, especially these days. Yet they stenciled this sign on the back wall of the ambassador’s residence on 16th Street downtown. But before we start chuckling over funny foreigners and their English, can someone tell us exactly what is the opposite of “formal entrance” in English? “Casual entrance” just doesn’t do it, though we understand the thinking behind it. Back door? Rear entrance? Service entrance? They don’t seem quite right for this Beaux-Arts mansion. Historically known as the Mrs. George Pullman House, it was once the Soviet and Russian embassy but now houses only Russia’s top guy in town.