I would like to know if another browser would show the original comment or the altered one. Because the comment shows up and is then altered after the fact, I would blame a translation plugin on the client side.
I could see a poor-quality plugin being fooled by a different-language version of YouTube and auto translating.
I met the same problem when some videos had bizarre, translated titles. Nothing as weird as the OP, though.
This one: https://www.youtube.com/watch?v=K8TS1Nnbc54
I see the title "Grandpa's Grub #2 in the Aude" in Firefox; but in Chromium the title is "LA POPOTE À PÉPÉ Épisode #2 Dans l'Aude à Saint-Gaudéric." which is probably right (all the videos in the series are French, in French and titled in French).
So there's definitely something weird going on, and it's breaking badly youtube. Google never was able to understand that I'm able to read several languages, thank you, and I don't want no stupid auto-translate, ever. Particularly when auto-translate randomly drops words (as in my video example).
Same problem on Ebay, and they have a much worse translator than Google, so that English product names get sometimes translated in funny totally unrelated words. I asked them if there's a way to turn off the translation and get only original English terms but alas no way.
The guy says it was an amalgam of his previous comments he'd made that day and I'm sure if you have a channel of his size, you're likely to have YouTube open for quite a while, and since it's an SPA, you could get all sortsa concurrency bugs or whatever.
With that said, I'd like to add the obvious "BIG if true"
I 'm sorry, but even if true, who cares about youtube comments?
I strongly doubt that this is on YouTube's end, however - not because I trust Google not to alter comments if they believe it fits their interests, but rather because I believe they could do it in a more subtle manner (test backlash by only fixing typoes at first, for example, and probably hide major alterations from users).
I'd be inclined to think it's a bug in some extension he's using, or Chrome's automatic translation feature is doing something it shouldn't.