The kid who sold you that $400 department-store suit the other day may have been selling refrigerators the week before. But at M. Lang Executive Attire, on Euclid Avenue at East 13th Street, you're guaranteed a salesman who's style-savvy. Owner Mike Lang will greet you at the door and, in his characteristic debonair style, usher you to the racks. The store's suits are all Italian-made; there are piles of silk shirts and ties, and even the casual wear looks sophisticated. Lang says $200 can give most men the best ensemble in their wardrobe; his finest suits cost around $800. Either way, you receive fastidious attention from Lang, whose fashion taste can make an insurance agent look like a tycoon.