"knowledge of language" is just that: "knowing" about "language". If I say: "Her knowledge of language is amazing!" I mean that I am amazed by how well she knows and/or understands language. Take these definitions for example: knowledge - facts, information, and skills acquired by a person through experience or education. language - the method of human communication, either spoken or written, consisting of the use of words in a structured and conventional way; any nonverbal method of expression or communication. So "knowledge of language" is the facts, information and skills acquired through experience or education about methods of communication (spoken, written and nonverbal). Hope this helps :)