nixos-config/system/locale.nix

{ config, pkgs, pkgs-stable, ... }:
{
  time.timeZone = "Europe/Berlin";
  i18n.defaultLocale = "de_DE.UTF-8";
  console.keyMap = "de";
  services.xserver.xkb.layout = "de";
  i18n.extraLocaleSettings = {
    LC_ADDRESS = "de_DE.UTF-8";
    LC_IDENTIFICATION = "de_DE.UTF-8";
    LC_MEASUREMENT = "de_DE.UTF-8";
    LC_MONETARY = "de_DE.UTF-8";
    LC_NAME = "de_DE.UTF-8";
    LC_NUMERIC = "de_DE.UTF-8";
    LC_PAPER = "de_DE.UTF-8";
    LC_TELEPHONE = "de_DE.UTF-8";
    LC_TIME = "de_DE.UTF-8";
  };
}