Well, in the comments section would be most logical. (Alternatively, it could be a default plugin, and you could turn it on/off that way.) But leaving it it where it is could work. I just didn't expect it. And now, I'm starting to wonder why I didn't see it before.
I understand why, programming-wise, it is named "Allow_comments," but when it's being displayed to the user, don't you think that underscore should be replaced with a space. Even a simple strreplace("", "", $setting) would probably work.
It used to be called Allow comments (without the space). That must have got broken when the multi-lilngual stuff went in and we did not notice. If it is a bug causing this I will get it fixed for 1.2. If it is an omission I am afraid it will wait until after 1.2 since new text to be translated is frozen.