I do not understand this issue. Locales (the title) and codecs (the message) are different things.  A Tk Text widget, with Idle's wrapper, is essentially a BMP terminal. That is one thing that makes Idle's Shell better than Window's command prompt.  An ascii codec would add to tk's limitations, not work around them.  #14304 does what I think is needed.

Pending some clarification of the proposal and benefits, I think this should be closed.