Specifically, in the case that we're inheriting a language setting
from some earlier preference or something and the language we're
inheriting is one that saves prefixes, and the current file being
opened does not match any of the possible prefixes, then revert
to the not-a-language language, instead of using the value from
the preference
Also: finish the removal of the EoPL language level from the
DrRacket langauge dialog, and clean up the 'get guidance' dialog
Please cherrypick this commit to the 5.2 release branch