English as a proper noun for the people of England, or the language which bears its name, should always be capitalized. It is a proper noun, after all.
In the phrase, the English language we have two nouns, with one functioning like an adjective to refine the meaning of the final noun in the group. If it wasn't capitalized you would call english an adjective. Capitalized, it is a pronoun and makes the phrase a compound noun. Just because it is functioning to refine the meaning of the terminal noun in the phrase, does not make it an adjective. It is still a noun, I think.
Etymologically, this usage is probably derived from a common language shortcut where people just started saying things like the dining room table, instead of the table of the room of dining, which is a sort of construction you might see more often in Spanish and Italian.