From my limited experience with people on the autism scale. I imagine that the grammar rules of English or any language could be mastered by an autistic person, but that mastering the speaking portion of learning the language would be very difficult. Autistic people often suffer from intense shyness and an inability to converse easily with others, limitations in the language could easily exacerbate that issue.

For example for them often is hard to recognize from somebody face that he or she is tired, angry, bored and it is hard for them to feel emphaty for other person

I think they are not shy, but have problems with understanding social relations, because much of communication between people is not verbal and they have also problems with unverbal and indirect communication.

As far as I am concerned, autistic people have problems with understanding metaphors. They think very logically and each sentence treat very seriously.