I was looking at job postings for interpreters and found a few for generic "Chinese" interpreters, a few more for both Mandarin & Cantonese, and just one for Mandarin only. To my surprise, I didn't find any for Taishanese, Min or other dialects. Am I right to assume that fluency in Mandarin and Cantonese is a de facto requirement to become a Chinese interpreter?
Probably depends on the locations and needs.
People who speak dialects probably can understand at least either Mandarin or Cantonese. That can be the reaons why Mandarin and/or Cantonese is the requirement in general.