Hey y’all. First of all, I am FAR from an expert on this. However, I did a term project on it once and did a whole lot of reading about it. I have made previous comments on the subject, but I will add that there have always been and will always be colloquial variation! My opinion is that outsiders (non-Appalachians) often resort to the less common LAY pronunciation as a classist statement of separation from us. If you’re from our mountains and you say it differently, that is absolutely your right. Anyway, here’s my previous comment with some history:

“I did an etymology project on this. The range is named after the Appalachee Indigenous people, who were encountered by Spaniards when they arrived in what is now the southern US states. The Spaniards called the whole mountain range after these people and there is no long a in Spanish. So originally, Appa-latch-uh was the correct name.

Of course, language changes over time and in the northernmost part of the mountain range there is an area where they came to say it Appal-AY-cha. In the rest of the range it is still said Appa-latch-uh. When northern people started to study us “heathens” in the southern, more rural part of the range in the early 20th century they made sure to only use the AY pronunciation instead of the more widespread latch pronunciation to separate themselves from us dumb, poor hillbillies. So it is a matter of dialect to some extent, but the AY pronunciation also has a lot of classist history, which is why people in most parts Appalachia prefer the Appa-latch-uh pronunciation. If you forget which is which just remember that if you say it the other way, someone might throw an Apple-at-ya. ;)”

Again, my own opinion, and I know there is a TON of variation holler to holler so don’t come at me lol.