Win8 single language to win8 pro upgrade

Try this
http://www.microsoftstore.com/store/msstore/pd/productI...
BUT it may well be worth looking for a standard 8 Pro Upgrade as many Retailers are offering large discounts compared to M'Soft Store prices. eg
http://www.ecostsoftware.com/microsoft/microsoft-window...
Unsure if that will work upgrading your version of 8 single language though...but if you have a copy of XP you could install 1st...