This strong dialect, almost a language type thing is pretty common in the UK in general. I grew up in the Welsh border counties of England, and it was pretty common amongst remote rural people to speak a dialect that would have been impossible to understand by people in the towns 50 miles away. Bits of it survive, in Shropshire it is relatively common to hear locals say the weather is "cowd", and to pronounce words like "sheep" as "ship" (the old phrase "spoiling the ship for hap'orth of tar", meaning ruining something of value for the want of a cheap bit of maintenance, is apparently not about boats).

I find most intriguing that men call each other "mon", like, "Surry mon it's cowd", or "how bin'ee mon". However the equivalent English, "hey man, it's cold today" would be absurd in England. To the old folks "she" was normally replaced with "her", as in "her is cawd".

So those are all variant pronunciations or usages, but it goes further. My father is an upper-wommer (a yokel, but more effectionately meant. Literally someone whose house is high up in the hills). When he spoke to his father I used to struggle to follow say all. They would talk of "tumps" (small hill), "unts" (moles), and the lovely "unty-tumps" (yes mole-hills). Owls were "ulerts", gaps in hedges were "glats" bill-hooks were brummucks. There were bits of old-English thrown in, I was once told a brummuck needed "whetting" (as in whetstone, sharpening stone) and was confused enough as a child that I thought soaking it in oil to treat the rust should do-it.

By the definitions above, if I as a teenager couldn't understand my grandfather (who I saw regularly) talking to my father, surely they were speaking a different language?

Btw despite this being the Welsh borders, Welsh is very uncommon as a language, and whilst I don't speak Welsh, the vocab doesn't seem to come from there. Welsh has a number of sounds like "Ll" that were not used.

Post WW2 farming became very prosperous in the area, and lots of farmers became gentrified and sent their sons to the elite English private schools (confusingly known as the Public Schools, for historical reasons). I think this is largely responsible for reducing the usage of the local language. Know you can still find it on building sites, farm labourers, and in little remote pockets.