Re: Language negotiation
Oops, I just found out that this functionality actually appears to be implemented.
I tried it using several VPN exit points.
It's in Store Setup -> Translations, Languages Tab.
There is a column "Countries", where clicking in the field behind each language, the countries, where to use this respective language can be selected or deselected.
My bad, if it is documented in a proper way and I did not read it, lack of documentation if it's not and I could not find it.
This solution looks good enough.