Re: rough sketch of a potential solution

>> The problem is that there may be multiple recipients and the sender 
>> may not know which recipients can read which languages, or even if 
>> this is known, the sender may not know alternative addresses for some 
>> of the recipients.  Consider a reply to a message that was originally 
>> sent to a list of recipients, some of whom the sender knows and some 
>> of whom the sender doesn't know.  Either the original message or the 
>> reply could be in multiple languages (say with multipart/alternative)
>
> Do you have information on how often multiple languages (with 
> multipart/
> alternative or plaintext, but nevertheless parallel texts) are used?

no, but the content of messages is not relevant here anyway; we're 
talking about the message header and envelope.  it's quite frequently 
the case that mail is exchanged between people whose names (thus email 
addresses) are in their native languages, and whose native languages 
and scripts are different.