nixos-config/config/i18n.nix

{ ... }:
{
  console.keyMap = "neo";
  i18n.extraLocaleSettings = {
    LC_ADDRESS = "de_DE.UTF-8";
    LC_MONETARY = "de_DE.UTF-8";
    LC_NAME = "de_DE.UTF-8";
    LC_PAPER = "de_DE.UTF-8";
    LC_TELEPHONE = "de_DE.UTF-8";
    LC_TIME = "de_DE.UTF-8";
  };
  i18n.supportedLocales = [
    "C.UTF-8/UTF-8"
    "en_US.UTF-8/UTF-8"
    "de_DE.UTF-8/UTF-8"
  ];
}