There is a Chinese supermarket in the US I often shop at. I often hear the staff speaking Mandarin or Cantonese, and occasionally Hakka or Fujianese, amongst themselves and to Chinese customers. I can also tell that they speak English to non-Asian customers. As a non-Asian, what is the best approach to take if I want to practice my Mandarin there? I know a little Cantonese, but my Mandarin is a lot better. Even if the cashier does know Mandarin, I am worried about offending them if I don't validate their proficiency in English by talking to them in English.