nixos-config/modules/locales.nix

{ ... }:

{
  i18n = {
    defaultLocale = "en_GB.UTF-8";
    extraLocaleSettings = {
      LC_MONETARY = "de_DE.UTF-8";
    };
  };
  console.keyMap = "de";

  time.timeZone = "Europe/Berlin";

  location = {
    latitude = 49.52;
    longitude = 10.17;
  };
}