Is that possible to understand some other dialects if a foreign student understand Chinese Mandarin very well?
I think if you know Chinese Mandarin very well , you also can understand most of Beijing dialect (北京话)and Northeast dialect（东北话）,but around 20% of them are still different from Mandarin . I also think Tianjin dialect(天津话). is an easy understood dialect. Maybe as a foreigner , if your Chinese mandarin is perfect , you can understand 50-60% of Tianjin dialect .