Note that this is often done based on your IP address if you've not opted-in.
It's often a quirk when using CCC (Chaos Computer Club) IP addresses at events in other countries; you'll be looking at google.de for the first couple of days until google correlates enough location data from people that have opted-in to correct the IP location.
On the plus side it means that a personal VPN with no opted-in devices will give you your home-region services (ymmv).
It is not only the UI language that changes, the search results do as well.
Suddenly something that was easy to find becomes a mess of local results with similar names or machine translated, even though I used the original language name.