that's a pretty fast timeline to have such a disjunction. usually it takes a few hundred years for languages to drift that far on the main branch. but that's looking backward, where we have the memory of the changes built into our modern forms. add to that the fact that we're seeing a lot of rapid linguistic changes in the contemporary era, largely bc of societal and technological shifts that are fracturing the language.

it's likely a person alive today dropped 100 years into the future at the same place would have some continuity that would make understanding possible, and certainly the future folk would understand them. still, I suspect it wouldn't come naturally or without study.

and all that goes out the window if they're dropped into a different cultural context. American English and British English are already diverging significantly, and we're seeing a bazillion dialects that are spinning off into the weeds. I can't understand a great many native English speakers' accents and dialects today without focusing really hard and immersing in it for at least an hour. so combine all that with the slang and tonal changes and you're in for a wild ride.