I wonder if there is any research conducted in China, on the relative difficulty of Chinese speakers learning different dialects. I'm looking for something like the research by the Foreign Service Institute (this is the relative difficulty of learning a foreign language for English-speakers).
I'm especially interested in how hard it is for Mandarin speakers to acquire Cantonese.
(This may not be a proper answer.)
I don't know if there is any study about that.
The relative difficulty is based on learners.
For nonnative speakers, they may find learning Cantonese is as difficult as learning Chinese. For Mandarin speakers who were born in the north of China, they may consider Cantonese as difficult as a foreign language. But for people who were born in Fujian Province, they may learn Cantonese easier than others.
There are a wide variety of dialects in China. Usually, people from the same region may understand each other's dialect well. But this is not true in Guangdong Province.
For most Mandarin speakers, they may find Northeast Dialect, Henan Dialect, Tianjin Dialect and Sichuan Dialect most easy to understand.
