[Resending. My original message was filtered out as being from a non-member.]
RFC 2184 provides methods to encode language information in message
Yes. The language community has serious problems with RFC 2184, however (as
in, it cannot be parsed and it breaks compatibility with existing practice),
and no one has implemented it that I am aware of.
From my reading it is a little unclear if it can be used for specifying
language in message bodies. For example:
Content-Type: text/plain; charset="iso-8859-1'en"
No, you would not use RFC 2184 for that; you would use RFC 1766:
Content-Type: text/plain; charset="iso-8859-1"