d'oh of course .. been a long time since I looked at those values.
As pointed out by Kanegasi the itemType and itemSubtype are numbers and are not needed to be localized.
The following demonstrates how to return a complete list of localized itemtypes as strings. If you put these into an indexed table and then use the GetItemInfo itemType to retrieve the string value it should ( in theory and if memory servers ) return a localized string of that type. Or the alternative keyed table with the localized string as a key returning the number that relates to that value. You may have to test it out to see if it will suit your requirements.
https://wow.gamepedia.com/API_GetAuctionItemClasses
It may help with retrieving info from the tooltip but that is not something I have personally needed to do so can't confirm there.