What about keeping accounts as they are, but letting users define links between
them?
For instance, EnUser on en: declares his user matches DeUser on de:, does the
reverse. And automatically when s/he logins on either of the sites, login occurs
on the other sites. Because the system knows that EnUser on en: <=> DeUser on
de: (obviously you need both declarations for the match to occur).
Ok that would still let users with different accounts on different wikipedias.
But you could retrive interwiki modifications from the user, using linked
information.
And it would let users keep their current accounts, so no friction.
OTOH, it means 'User' on en: is not necessarily the same as 'User' on
de:.
Just my 2 cents of €
Nicolas