I have finally found out how to request the connection between the Hungarian Spelljammer Wiki (hu.spelljammer.wikia.com) and the English Spelljammer Wiki (spelljammer.wikia.com). It was hidden in the posting instructions on the page I had looked at for interwiki requests:
Community Central:Interwiki requests (Spelljammer Wiki)
Sorry for the delay. The good news is I'll be able to do it straight away, next time.