It's really not that simple. Merch is mostly a south walian word but it is commonly used on the east of Anglesey in the north. This unit is more a way to show people the duplicate words since narrowing down each word to one dialect is reasonably difficult. For example I am happy to use "Merch", "Hogan" and "Geneth" for girl but my dialect is technically North.
As I was brought up in NE England, with a mostly Yorkshire family, had a good education and the lived in the English Midlands and NW my English is full of idiosyncratic vocab and pronunciation. Occasionally Southerners are puzzled (or even annoyed) by my northern vowels, but they never notice the inconsistencies. I doubt the Welsh are less tolerant than the English on this, especially with non-native speakers.