While nearly all Shenzhen-area residents speak Mandarin, only some speak Cantonese
Over 85% of Shenzhen residents come from a province outside of Guangdong (Guangdong is the province in which Shenzhen is located). Even the poorest of the poor factory owner, even one with no education at all, can speak a good amount of Mandarin. Although many businessmen and women from Guangdong province may speak Mandarin with a bit of an accent (in fact they do so with one stronger than mine), they all grow up going to schools where the working language of instruction in all of their classes (whether it be 4th grade math, or 10th grade history) is Mandarin Chinese. Also, with a few minor exceptions, the Chinese characters used by both Mandarin and Cantonese are the same.
© Rob Stark, 2016; I can serve as your Chinese interpreter/translator in the following cities and provinces in China: Shenzhen, Hong Kong, Guangdong province, Dongguan, Huizhou, Macau, & Foshan.