I have two teenage daughters, and they both have severe autism, with IQ’s that were estimated to be less than 50. They are not yet able to learn language and related skills from their environment. Everything has to be taught in tiny steps with lots of repetition. Every word they know to say or sign or understand has been painstakingly taught in very labour intensive one to one teaching.
As they learn more, I hope that they can start to pick up some of their learning from the normal environment. My older daughter doesn’t talk at all, but is learning to use British Sign Language. She now has over 300 signs. She also learns to read the word card, and match it to a picture, for each new word. So she is able to make requests using a sign or word card or picture, though she strongly prefers signing. My younger daughter has some spoken language, and has learned a comparable number of words, but her pronunciation is sometimes poor. Both girls have a better understanding of spoken language when it is combined with signs or other visual information.