"Graham Murray" commented
... Therefore, ideally it should be in the same language as the
rejected email - but I suspect that it would be impossible (unless
the sending MUA has included, as some web based MUAs do, an
'X-accept-language' or similar header) to determine the language of
the rejected email and return the reject message in that language.
Michel Bouissou has understood the way I was thinking.
The intended recipient will want to make her own choice of the language to be
shown to the senders of legitimate messages which are bounced. If she knows
that the majority of people who legitimately send here eMails speak Japanese,
she should be able to use that language for her rejection message.
Adding multiple, alternate languages, selected with reference to the message
being bounced, as you suggest, is, IMHO further than we can go today with the
available technology.
Chris