US English comes to mind, not that middle English is particularly readable.

I was thinking of English as a counter case when I wrote the comment:) Skimming the history section of https://en.wikipedia.org/wiki/English_language#History , I don't think I would really describe anything before "Early Modern English" (1500-1700) as meaningfully the same language; if you took a modern English-speaker and gave them a sample of Middle English, they wouldn't be able to read it, and if you stuck them in a room with a speaker of ME neither would be able to understand what the other person was saying.

Although in fairness, it does now come down to a semantic argument about what counts as the same language, and I acknowledge that a reasonable person could disagree with my narrower view.

"Englischmen þeyz hy hadde fram þe bygynnyng þre manner speche, Souþeron, Northeron, and Myddel speche in þe myddel of þe lond, ... Noþeles by comyxstion and mellyng, furst wiþ Danes, and afterward wiþ Normans, in menye þe contray longage ys asperyed, and som vseþ strange wlaffyng, chyteryng, harryng, and garryng grisbytting."

This is...difficult to read, but once you realize that þ == th, it's semi comprehensible. Looking up some Middle English on Youtube, it's also semi comprehensible. I doubt I could have a deep philosophical conversation with an ME speaker, I think we could make eachother understood.

> Although in fairness, it does now come down to a semantic argument about what counts as the same language, and I acknowledge that a reasonable person could disagree with my narrower view.

I really enjoy Jamaican patois for this. Arriving there, try as hard as I could...I couldn't understand a single word of it, even though it was ostensibly "English-enough" that I should have been able to. The first 5 days spending time in groups speaking the local language felt like anywhere else that I couldn't understand -- Saudi Arabia, Portugal, Guatemala, etc.

Right around the one week mark, something just 'clicked' and I could understand pretty much all of it as if it were regular English except for the true slang. Really felt like "dialect" on the cusp of become "language". Very cool spot for a language.

> What language is over a thousand years old and still recognizably the same language?

Latin.

This answer wins. I suspect this is the most widely-used language that has not changed in 2000 years. When I was at school in the 80s we were seemingly learning the same Latin as spoken by the Romans.

Arabic

Icelandic and old norse perhaps