I'm looking at travelling to Shenzhen for a month. Which dialect(s) should I study? For speaking with factory managers, workers, and for day to day life?
Shenzhen is the United Nation of China -- take the subway and in any car you will hear a dozen languages and dialects. The whole of China is represented there -- from Guangdong people, of course (not all of them being fluent in Cantonese, there's a whole bunch of Guangdong-born Hakka and Chaozhou people in Shenzhen) to neighboring Guangxi (lots of them in Shenzhen and Dongguan; they were the first to settle down in Shenzhen when the FEZ was opened. They speak a moderately intelligible dialect of Cantonese, good luck with that) to Fujian, Hunan, way up north (always funny to hear a taxi driver asking 去儿哪儿里儿?) and even Uyghur people... Nobody speaks Mandarin but everybody think they do :-)
Besides, how long do you have before your trip? A couple of years?