The aim is to let interwiki links like [[en-simple:Something]] work (i.e. return the same as [[simple:Something]]), since en-simple is the language code (correctly) returned by the Wikidata query service.
It's based on trying to make a change in links in the query service (see linked on-wiki request). As I don't know if that'll blow up anywhere else in WD I'm trying to double check. If not that's cool.