nix-conf/system/locale.nix

{
  config,
  lib,
  pkgs,
  ...
}: {
  i18n.defaultLocale = "en_US.UTF-8";
  i18n.supportedLocales = [
    "en_US.UTF-8/UTF-8"
    "ja_JP.UTF-8/UTF-8"
  ];
  i18n.inputMethod = lib.mkIf config.u.has.graphical {
    enable = true;
    type = "fcitx5";
    fcitx5.addons = with pkgs; [
      fcitx5-mozc
      fcitx5-gtk
    ];
  };
  fonts.packages = with pkgs;
    lib.optionals config.u.has.graphical [
      noto-fonts
      noto-fonts-cjk-sans
      source-han-sans
      source-han-serif
      nerd-fonts.jetbrains-mono
    ];
}