How to say "slang" in Cantonese and Mandarin?
For "slang"

俚語- Mandarin

俗称- 'slang' (n) Cantonese e.g.「孖煙囪係男裝短內褲嘅俗称」('Double chimney' is a slang for men's short under pants)

俗称 (v) 'commonly called' e.g, 「男裝短內褲俗称孖煙囪」 (men's short under pants is commonly called 'Double chimney'

俗语 = 'folk saying/ as the saying goes' e.g. 俗語都有話：「牛唔飲水點㩒得牛頭低」( As the saying goes: " If a bull doesn't want to drink water, there's no way you can force its head down")