Of course Mandarin is a written system. Pop music from Hong Kong is usually in what basically amounts to Cantonese-pronounced Mandarin. That's why they sing 是 i

Of course Mandarin is a written system. Pop music from Hong Kong is usually in what basically amounts to Cantonese-pronounced Mandarin. That's why they sing 是 instead of 系 and it's why people recommend NOT using pop music as a learning tool when learning Cantonese. Written Cantonese also exists: 係唔係佢哋㗎？ vs 是不是他們的？, for a Wikipedia example. The first is what people speaking Cantonese actually say, the second is what they might write if they're trying to be formal or official and it's probably what you're used to seeing.