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)).
grin