I'm sorry but "For the moment, this case is just not supported." is not
an explanation of why this is not a bug.  It is a statement that the
interpreter cannot handle a situation that has arisen.

If you said, "We don't believe that any computer has mixed encodings
that can show up in environment variables" that would be an explanation
of why this is not a bug and I could then give counter-examples of
computers that have mixed encodings in their environment variables.  So
what's the reason this is not a bug?