Installation :: How To Find Which Was Original Language

I recently tried to upgrade a friend's Windows 8.1 notebook to Windows 10. I had a Windows 10 Greek DVD-ROM with me to save time waiting for the update to download. I checked System Properties, it was Windows 8.1 64-bit, everything was in Greek, so I assumed he had the Greek Windows version and I could proceed with the upgrade using my DVD-ROM. Unfortunately he originally had the English version and he had added the Greek Language Pack and changed the default display language to Greek! So after wasting about half an hour for Windows Setup to check and download updates, we were presented with the choice of keeping his data only (second), the choice to keep both data and applications (first) was grayed out. I read at the bottom that I could not make a proper upgrade because the language I'm trying to install was different from the system language! Oh my God! Eventually we wasted another hour to wait for the upgrade to download!

So the question is how can I check which is the original system language? One possible way is to install a utility such as Aida64 and go to the Operating System section, but this is not really convenient to download and install a utility just to check the language version. How can I do it from Windows without using any utility? System Properties do not display this information and if the user has changed the display language, like in my friend's case, you can be deceived. Neither does winver, I checked.