The langAutomatic doesn't have to be a MIME type, necessarily.
If you want to, you can define an "application/printable", but
just saying that it's legal to ship "application/octet-stream" to a
printer and let the printer figure out what to do with it would work
just as well.
In E-mail currently, application/octet-stream is used heavily to ship
documents in "let the recipient figure it out" mode.