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 .