Wiped 6s need operating system restored

There is an operating system (iOS) on it when you turn it on. You cannot get rid of iOS. It is iOS that asks for language and region. That's why the answers you are getting don't make sense to you, because you think the operating system is not on it, but it clearly is if it is asking for language and region.

Your problem is that it cannot activate, not that there is no operating system on it. And iTunes is seeing your phone if it says it can't activate it. To activate your phone it must have a SIM card in it and either an active cellular service account or a Wi-Fi connection. So this is the link you need-->If you can't activate your iPhone - Apple Support

You can also just click the Restore iPhone button in iTunes. That will put a clean copy of the operating system on the phone, overwriting the one that is on it already.

Follow the instructions in this link-->If you forgot the passcode for your iPhone, iPad, or iPod touch, or your device is disabled - Apple Support

Try https://support.apple.com/en-us/HT201269

What do you mean by restore Operating System? You cannot downgrade iOS.

That link doesn't restore iOS. You may have meant a different one.

Right, so click on the Restore button to do so.

Follow the link that I posted. Click on the blue text to open the link.