Sorry, I cannot parse "English language and usage", at least, a similar phrase in Russian would be ungrammatical.
In the phrase "English Language and Usage", "English" is most definitely an adjective. The reason you're confused is because the phrase relies on implication.
Rephrase as "[the] English language and usage [of the English language]" and It should be clearer this way. The ending preposition is not used because it is overly verbose and somewhat redundant.
The word "English" can be both an adjective (as in the "English football team") or a noun meaning either the people of England or the language spoken there. "The English often drink tea at 4pm." "He speaks English very well."
In this case it refers to the English language. The specific usage here is called apposition, where two nouns are put together to form a genitive construct. So "English Usage" means "the usage of English [the language]".
Here is an article on apposition you might find interesting including its various forms.