I have a number of personal anecdotes on the subject, which I will share FWIW.
Once I had dinner with two other people I know well. One was Serbian and the other Georgian, the American state. The latter had grown up partly in Atlanta, partly in Savannah. The G asked S (who had lived a couple decades in the US) what the difference was between Serbian and Croatian. S sighed and said, “I have been out of that milieu long enough to admit the truth. In fact the difference is less than the difference between your two accents.” I’m from Philadelphia, which is where S had been living for at least 20 years.
I have twice been in London and not able to understand a word of what someone was station. I was trying to ask a porter in Victoria (I think) station where the boat train to Paris was leaving from and I could not understand his answer. I asked him to repeat this and it remained incomprehensible. Although he seemed to understand me. The other was a gas (sorry petrol) station in London.
I once met a highland Scot whose English sounded perfectly normal Scottish accented English. But then he gave me a sample of the English (definitely not Gaelic) that he would speak to his family and it was incomprehensible.
I have been to Barbados every winter (with one exception) since 2001. They speak what they call English and their radio announcers are perfectly comprehensible. I suppose they are imitating BBC English. And they can almost all speak perfectly understandable English to me (I do recall one exception). But their native dialect is Greek to me.
My colleague who grew up in Germany until he escaped at age 16 in 1939 spent a year in Zurich in 1965-66. He said it took him till the end of that year before he really understood Schwyzerdeutsch. But the government insists that schools be taught in Hochdeutsch starting in 3rd grade. When my friend from Georgia spent a year in Zurich, his kids were speaking Swiss in a month or so, but said that German was “too hard”. Clearly they picked up the playground language not the classroom language.
I can understand, more or less, Parisian French when spoken slowly enough. Quebecois French defeats me, for the most part. Even though I have been living here for 46 years. Living in English, obviously.
Finally, this is crime fiction, but the authors were a pair of Swedes. The murder took place in Malmo, Sweden, but the perp escaped to Denmark. So the chiefs of police of Sweden and Denmark had to meet to discuss the case. According to the fiction, now that they had something substantive to discuss, they had to forgo the pretense that each understood the other’s language and speak English.
The conclusion I draw from all this is that language boundaries are confusing and ill-defined.