nix-conf/system/locale.nix

{pkgs, ...}: {
  i18n.defaultLocale = "en_US.UTF-8";
  i18n.supportedLocales = [
    "en_US.UTF-8/UTF-8"
    "ja_JP.UTF-8/UTF-8"
  ];
  i18n.inputMethod = {
    enable = true;
    type = "fcitx5";
    fcitx5.addons = with pkgs; [
      fcitx5-mozc
      fcitx5-gtk
    ];
  };
  fonts.packages = with pkgs; [
    noto-fonts
    noto-fonts-cjk-sans
    source-han-sans
    source-han-serif
    (nerdfonts.override {fonts = ["JetBrainsMono"];})
  ];
}