Regarding English variants not recognised by MediaWiki (en-AU, en-NZ, en-ZA, en-IE, etc): they're more-or-less synonymous with en-GB aside from a few orthographic edge-cases. They should map to International English (although here in Australia, -ise suffixes are the preferred spelling), and definitely not en-US (and by proxy, en, assuming it implicitly defaults to en-US).

FYI, I was the one who removed the (ostensibly) redundant en-gb label @Nikki spoke of. I find this complication to be emblematic of a more broader issue—the need to distinguish regional variants from an ultimately "neutral" variant (which in English's case, is unfairly assumed to mean en-us). It would be nice if regional variations were defined explicitly, perhaps laid out like this: