I think that's true however you slice it. There's lots of possibilities for defining the default: but the language the user has chosen to use for their system i

I think that's true however you slice it. There's lots of possibilities for defining the default: but the language the user has chosen to use for their system is vastly more likely to be right than a best guess. The other one is: pick to one language and stick to it. If the page I'm reading is in some European language, please show the GDPR messages in that European language not some other language. If it's a French blogspot blog being visited by a user in Germany, show the messages in French! This is almost guaranteed to be 100% reliable. It's not like Google is ignorant of the language of the page. (I can understand not having GDPR messages translated into say Indonesian. But if you do, then again - match the language of the page.)