So I'm from England myself and my question really is, I guess, how far back in time would I have to go that I would struggle to converse with a fellow Englishman? And what were the notable milestones for the language changing significantly?
There's always space for new takes on a topic but while you wait you could check this post by
/u/bloodswan from the FAQ. I'm sure I've seen other answers that haven't been swept up into the FAQ so some careful Googling might turn up other results from the sub.