You do indeed need to change it from all those wikis.
It is typically
something that a bot does well, so the best thing to do (if there are
more than a handful of languages involved) is probably to contact a
bot owner and let them do it. I have taken this message as such a
request, and resolved the Ferenc Farkas issue.
However, whether done by bot or manually, there is still the problem
that there might be a bot that sees some of the languages before, and
others after the change; such a bot would consider the interwiki a
'missing' one on the languages coming afterward, and re-add it. I
don't know of an easy way to resolve this, the only thing I can think
of is adding an article on the composer on eo:, or on the esperantist
on one of the other languages. Or keep an eye at the articles, and
keep changing until we get lucky...
There is a method keeping bots away from putting a wrong iw to a single page by commenting
out the wrong link like
<!-- [[de:Bad Link]] -->
But this does not prevent the bots from putting on others sites pages.