Yes, they do, BUT you can just make them non-writeable by "diradmin" so the update won't overwrite the images directory or something. If you let everyhting else update, you'll have a current skin, with your own buttons.
It would depend on what you mean by "buttons" .. I was thinking the gif images. But if you're referring to the <input type=submit> type buttons, those are generated by DA, so they are not in the
Yes, error messages are created by DA.
We are chewing on the idea of adding languages.. It would be a major overhaul, and the language files would be massive (each operation might have 10 possible error messages, so multiple that by however many things you can click...and you've got yourself a lot of work