Quite true, but I only think it occurs on the quote tag, since it has the language dependent string "In reply to:"
I thought this would occur in a standard threads installation as well, but it is not. The language variable is written to the post so that it will show according to the user's settings.
In the MarkupHack I do an eval of all strings before they are inserted into the post. Can't remember why this was needed now though, I must have had a reason for it. I will have to do some checks on this to see if I can solve it.