Adding Language Packs is not the same as having the O/S in a different language.
I followed this Tom's Hardware article and found some links on the MS site. https://www.tomshardware.com/reviews/get-windows-10-free-or-cheap,5717.html
The links are in the following paragraph on Tom's page.
"No matter how or whether you pay for it, you can download Windows 10(opens in new tab) or download Windows 11(opens in new tab) for free from Microsoft.com. And, in fact, you should only download it from Microsoft, as grabbing it from any other site or from a P2P network could give you malware. Microsoft offers a free media creation tool, which grabs the latest code from the Internet and then burns itself to a USB Flash drive or outputs a Windows 10 or Windows 11 ISO file you can write to a drive yourself. "
I was able to remove a checkbox from the "recommended version" which must have sniffed my English Config and I was able to select to download any Language Version, so I chose Spanish. This gave me the .iso I need.
Starting the Hyper-V installation it gave me a choice of many versions: Home, Professional, N and S etc. I only want to debug my app briefly and then I don't need the Spanish OS anymore. I was able to bypass the "licence-key" entry by just clicking "I don't have a licence key" (in Spanish :)) I can login and use Windows.