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.

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.