One question worths investigating is whether there is any case when links
are not desired to be two-way: Article A1 links to B2 but B2 doesn't want to
link back to A1 (or wants to link to A2 instead). I don't know whether there
is such a case, maybe someone of you knows one.
I believe we use en: as the planned interwiki repository: if a link gets
into en: then bots propagate it around. Bad links in en: propagate, too. If
there was a single repository of interlanguage links these errors could be
fixed easily. Now it's pretty hard, even with scripts and bots; usually
needs a natural unintelligence machine (a Human Being(Tm)).