I suspect an add-on, because they are often the cause of any issues, but also because I recall seeing either a settings page or help page from one of them be in German. I'll have to look through my list of add-ons to see if I can find which one it was. Then I could check the source code to see if there's anything that might occasionally be overriding my normal settings.

Thanks for the links. Right now, the Google language settings shows English selected. When I have the problem again, I'll check if it was changed. But I don't think it is Google-specific, due to general web error pages sometimes being shown in German too. I think my browser is sending some flag saying "show me stuff in German!"

Truth be told, it's been more a source of amusement for me so far, than an actual problem.

I haven't been using any VPN, by the way.