How can you alter the display of Flag confirmation dialog messages?
I figured this out.

When you create a new Flag, at the very bottom of the Flag set up screen is the option to choose if a dialog confirmation box is required for selecting your flag.

If you choose yes, an additional option box becomes available below this were you can set the message to be displayed when a user is either Flagging or Unflagging content with this dialog box.

If you don't enter any values (text string) in these fields, it seems the default text input by the system is "Flag confirm".