ON MY FIRST NIGHT in Paris to see the fall menswear collections last week, I passed a young man wearing a weathered cowboy hat, a checkered flannel shirt and a pair of “how can he walk in those?” skinny jeans. It was the sort of outfit that would look more at home in a Los Angeles coffee shop populated by ironic Silver Lake hipsters than a side-street off the ritzy, historic Place Vendôme, so I assumed he was American. Yet, as I got closer, I heard him jabbering away in seemingly native French to his female companion. In a shin-skimming black coat and slim pants, she was the very picture of “French style,” one of the...