[larkeith]

That it's YouTube is utterly irrelevant - if this is not an external factor (e.g. a clientside translation plugin, as suggested elsewhere) it sets a very dangerous, and likely illegal, precedent, even more notable from being a Google-owned site.

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).