TRANSLATORS TEAM (Not inclusive) ...

While written forms of Mandarin maintain a rigid standard across the country, the dialects of PRC vary widely in their pronunciation patterns....
Thus, if your goal is to be widely understood, they should learn Mandarin because Mandarin can be understood even in Hong Kong, Macau & Canton (the main regions who still speak Cantonese)...
There are a lot of Chinese dialects in PRC, so a lot of that it is hard to guess how a lot of dialects actually exist...
While Chinese dialects have a standard in the written form, Cantonese can be written as it is spoken, although it can also be written in standard written Chinese. ....

Both Cantonese & Mandarin are tonal languages, but there are up to nine tones in Cantonese compared to only four in the main Chinese dialect of Mandarin. This makes Cantonese a more difficult dialect to learn...