Programming language design for humans is a completely different thing from PLT. It's more about usability, HCI, psychology. And if by success you mean populari

Programming language design for humans is a completely different thing from PLT. It's more about usability, HCI, psychology. And if by success you mean popularity or any metric that includes productivity of the language, understanding of usability will definitely help you there, but knowledge of PLT will almost certainly hurt it, since it makes you think that a lot of PLT ideas are acceptable and usable.