Hi Albretch,
If I'm reading your question correctly:
Why can't you set the locale for one country, the timezone for a second, the keyboard for a third?
You can. As root or equivalent, you can reset timezone with
dpkg-reconfigure -plow tzdata
[That's the dpkg-reconfigure command, -plow to force asking low priority questions rather than taking the default answers, and tzdata being the file that sets the timezone.]
dpkg-reconfigure -plow console-setup
[dpkg-reconfigure -plow and then the console-setup program. That might not be installed by default, so you might have to apt-get / apt install console-setup which will reset the keyboard layout that you see in a terminal - UTF-8 is usually a good choice to start with.]
dpkg-reconfigure -plow locales
will set the system wide language and spelling defaults etc. using the locales package and allow you to switch between locales
If you want to be asked all the questions at low priority during the install, consider using the expert install method which will ask _all_ the questions that a standard install sets automatically - if you set your locale to British English during the install, it will provide England/London or GMT as options for timezone and your keyboard to British UK layout, for example.
Live well
Andy C.