One can make an excellent case that AAVE (Ebonics), Bayou/Cajun English, Deep South English, Appalachian English, New York English, Newfoundland English, and of course Jamaican creole and Scots are separate languages. Even Scottish English and Geordie probably qualify.
A recent study found only 54% intelligibility for Standard English speakers of Geordie. The speakers were L2 English learners in the Czech Republic, but they scored 100% on the “home” test, which was a test of a US television English. Another study found 42% intelligibility of Scots for native speakers of US English. Having heard Hard Scots spoken by the Scottish underclass, I would say my intelligibility of it was ~5-10% at best or possibly even less. It was almost as bad as listening to something like Greek, and one got the feeling listening to it that you were actually listening to some foreign tongue like, say, Greek.
At any rate, 42% and 54% very well qualify both Scots and Geordie as separate languages. Scots is already split, and it sure would be nice to split Geordie, but to say people would get mad is an understatement.
Scots and Jamaican creole are already split off. There is a lie going around the intellectual circles that it is still controversial in Linguistics whether Scots and Jamaican Creole are separate languages. In fact it is not controversial at all.
I have been listening to English my whole life as an American, and I still cannot understand Bayou speech, hard Southern English, Newfoundland English or the hard forms of Appalachian English or New York English. There are some very weird forms of English spoken on the US Atlantic coastal islands that cannot be understood by anyone not from there, or at least not by me. Gulla English in South Carolina is already split as a creole.
Generally the criterion we use is mutual intelligibility. Also if you can’t pick it up pretty quickly, it’s a separate language.
A speaker of hard New York English came to my mother’s school a while back, and no one could understand him. They still could not understand him after three months of listening to him – this is how you know you are dealing with a separate language. He finally learned how to speak California English, and then he was understood.
I have been listening to hard British English my whole life, and I still cannot understand them. I even had a British girlfriend for 1.5 years, and I still could not understand her on the phone. She went to my parents house for dinner, stayed a couple of hours, and my brother said he didn’t understand a word she said.
You can make an excellent case that the harder forms of British English (or Australian English for that matter) are not the same language as US English. The problem is that if you tried to split them off, everyone would go insane (including a lot of very foolish linguists), and there would be a wild uproar.
Generally we use 90% as the split between language and dialect. Less that that, separate language. More than that, dialect. We use this criterion to split languages from dialects everywhere, yet if we tried to do it for English, the resulting firestorm would be so ferocious that it would not be worth it, but it would be perfectly valid scientifically. Even the very well-validated split of Scots has driven the English-speaking world half-nuts.
I actually have a post in my drafts where I split English into ~10-15 different languages, but I have been terrified to post it. My post splitting German into 137 different languages did not go over well with the Net linguists (who are mostly loudmouths, fools, cranks, and idiots), although a major Germanist, a professor at a big university in Europe wrote me when I was only at 90 languages and said, “I think you are right!” Still, if I try to split English, I may ignite one Hell of a damned firestorm, and I’m just too chicken.