We all know there are obvious difference between "Ye Olde English" and today's English, but at what point in time does it get so different that you wouldn't be able to understand anyone, even though you're both speaking "English"?
hi! not discouraging anyone from adding more examples from different times & places, but fyi, this is actually a pretty common question in this sub, so we have a few posts archived in the FAQ - check them out!
if you have follow-up questions on locked posts, ask them here & include the user's username to auto-page them