Technically a writing system is not part of the language It is separate Many languages do not have a writing system and many native speakers of languages that have one dont know how to use it In fact writing has only been invented a very few number of times if not just once in the history of humanity All others are derivatives But the main difference here is that writing has to be invented It is an addon to language and separate from the language itself For example a native speaker of English would never mispronounce the word their but might frequently misspell it In a generative view of language this is because the language is acquired naturally whereas writing has to be taught