Interesting. I experienced it differently myself (while I was traveling, both Google and Facebook would show me local language, regional sites), as have others we've tested with. We've done a bit of research into how others do it to come to this point, but it looks like we should dig a bit deeper.

[Accepted-Language] in the HTTP header and the location based on the IP address (Google has a super easy API for this) is the easiest way that I could think of. Both are pretty easy to implement as I have done both before.

This looks to be exactly the behaviour the OP is complaining about. Personally I think this is the correct behaviour.

unless you are on a VPN. then what ?

You will get localized date formatting among a few other things probably. I am just saying you could use location from the HTTP header that is meant to tell the server what languages you speak. The VPN portion could really only be fixed by allowing you to select and save an address that is originally defaulted to IP.

Out of curiosity, what happens when you go to this page in a private window:

https://productforums.google.com/forum/#!forum/en

Note the "en" in the URL and "English" in the language selector in the top right of the page.

The button text and input placeholder text is not in English for me, nor is the text at the bottom of the page. Blogger, Google groups and I think other Google stuff, like Flights, is the same.