How different languages interact is always cool :). FWIW it's not just songs; standard Mandarin is usually the prestige dialect/language so anything that's per

How different languages interact is always cool :). FWIW it's not just songs; standard Mandarin is usually the prestige dialect/language so anything that's perceived as being more "prestige" than an everyday conversation (e.g. things that are meant to be formal or official or whatever) tends to become Mandarinized among the different dialects/languages. Cantonese is a special example because of Hong Kong's own interesting social environment. For most other dialects/languages in the PRC, speakers just switch directly to Mandarin where they might have used a "Mandarinized" part of the dialect/language. Hong Kong is one of the few places where Mandarin comprehension is still not near-universal and so instead of just having Mandarin pop songs, you end up with Mandarinized Cantonese pop songs.