Baby Sign will NOT stunt the development of spoken language!
Late-talking children often develop their own sign systems that are understood by their families. Such systems should be encouraged so that the child develops enough key signs to communicate his/her basic wants and needs. This will greatly alleviate the frustration felt by the child while he/she navigates the outside world without a spoken language.
Remember, Baby Sign will NOT stunt the development of spoken language. Rather, it will enhance it by affording the child the opportunity to learn about communicative function (or pragmatics) before he/she is able to verbalize. But once spoken language emerges, he/she will quickly abandon signs for words, as speaking is a much easier task for a child who is surrounded by language speakers rather than signers.
Whether or not your child is late-talking, baby sign can only enhance the development of language concepts that form the foundation for later spoken language.