Currently, there is no way for users to delete or edit chat lines on online-go.com (it is possible on forums under some circumstances as those are running on a different software altogether).
If you need to hide any personal notes, or private info you have shared by accident, feel free to report the game, moderators can delete the line if needed. Other than that, there is no way currently.
Not sure I understand the comment about Chinese coordinates? The page should automatically create a clickable mark when sharing a coordinate, which should be universal for all as far as I know. Is that not so?
I think it’s probably the idea that if you write N12 in chat, there is no N12 in chinese coordinates. I think there’s two types of numerals instead like ニ12. Then if you have the chinese coordinates on, you don’t know which is which in the alphanumeric numbering/ordering to even write something like N12 to start with.
To the OP, one thing you can do though
CTRL+CLICK on stone or intersection (ALT+CLICK or CMD+CLICK on mac) – pastes co-ordinates of clicked stone into game chat. (Co-ordinates such as A20, A0, pass and U5 etc also exist by clicking the expected place)
That will paste the coordinates of the point in chat, and you can do it while typing. That will show up as @AdamR mentions. When you hover or click it, it should show the point on the board marked.
That makes sense. I thought it would work in reverse and also auto-create the link if you typed ニ12 if that was your board view, but I guess that’s not the case. My bad. Might be something to consider adding though