Wikimedia Bugzilla is closed!

Extension:InterwikiMagic adds global interwiki table support alongside local. Currently InterwikiMagic requires a patch be applied to this for this to work with it, as such multi-table functionality is simply not at all supported by Interwiki. This makes it messy.

Since both extensions are pretty much required if you want a good (flexible) farm setup that doesn't have the interwikis hardcoded in some way, this is bit of a problem.

(Wikimedia and Wikia use other methods, but besides ShoutWiki, other farms I checked don't tend to have global interwikis at all and this would probably be their best bet as well, if it worked.)