nixos-config/common/locales/fr.nix

{
  time.timeZone = "Europe/Paris";

  i18n = {
    defaultLocale = "fr_FR.UTF-8";
    extraLocaleSettings = {
      LC_ADDRESS = "fr_FR.UTF-8";
      LC_IDENTIFICATION = "fr_FR.UTF-8";
      LC_MEASUREMENT = "fr_FR.UTF-8";
      LC_MONETARY = "fr_FR.UTF-8";
      LC_NAME = "fr_FR.UTF-8";
      LC_NUMERIC = "fr_FR.UTF-8";
      LC_PAPER = "fr_FR.UTF-8";
      LC_TELEPHONE = "fr_FR.UTF-8";
      LC_TIME = "fr_FR.UTF-8";
    };
  };
}