What an interesting question! Let's begin by broadly defining language. Language is defined as a method of communication. A limited definition of language is a method of communication consisting of written and spoken words. A more limiting definition adds to this the idea conventions or language has a specific structure. For example, rules of grammar apply to language providing a structure.
Literature is a form of written language. In a strict sense, for written words to be called literature, the work has artistic or lasting merit. For example, The Great Gatsby is considered literature as it has stood the test of time. Themes in the book and the characters still have meaning to contemporary society even though the book was first published in 1925. Although there is some agreement as to which works belong to the canon of literature, it is easy to see how there can be some debate as to what belongs and what doesn't belong!
So, language is part of literature as language is the written word expressing the themes of the work. Here is the conundrum. Over time language changes. If you read The Canterbury Tales or any work by Shakespeare, this is an obvious statement. The conventions of language are not the same when the work was written as it is today. The themes are transcendent and this is what precisely separates literature from simply a good book.
One final thought on language. Earlier I mentioned broad or strict definitions of language. Language is far more than words. The expression of language comes in many different forms and combinations. When someone is speaking directly to us we interpret the language we hear and the subtle body language of the person speaking. Both contribute to our understanding or comprehension of what the person is communicating. This is why when we send what we think is a harmless text message to a person the person on the receiving end sometimes is offended by the message. They do not have the advantage of interpreting the body language with the text. What we think is harmless without the context of interaction can be easily misinterpreted.
In my view language is more complex than oral or text with strict conventions. The power of literature is the text most often precisely mimics face to face communication and elicits an emotional response to the communicator.