Interesting question. I was told by a professor that if we traveled back in time we would probably not understand a word our ancestors said. I tried it with some copies of written manuscripts and while I could get the gist of the writings I was a stumbling fool trying to read them aloud. LOL
Try listening to the epic poem Beowulf spoken in Old English. It sounds epic but I cannot understand a damn word of it. Listening to it it makes me feel like I had a stroke and lost my English, it sounds so familiar like you should understand it but you cannot.
I was in Norway at a language school and we went on a hike. Came across an older guy (72 years old) that lived up in the high valley we were in. The Norwegian instructors could only understand about 1/5 of what he said. (This was 40 years ago).
They said each valley had almost it’s own language as there wasn’t much travel between them. Of course that is going away now, but this older guy (would have been born in 1905 or so) was part of that history growing up.
I spent some time in the mountains of North Carolina - and some of the folks there I could only understand maybe 70% of what they said!