some refactoring and modularizing locale

modules/nixos/system/locale/default.nix

@@ -0,0 +1,64 @@
+{
+  config,
+  lib,
+  ...
+}: let
+  inherit (lib) mkIf mkOption mkDefault;
+  cfg = config.nixos.system.locale;
+  defaultCategories = [
+    "LC_ADDRESS"
+    "LC_IDENTIFICATION"
+    "LC_MEASUREMENT"
+    "LC_MONETARY"
+    "LC_NAME"
+    "LC_NUMERIC"
+    "LC_PAPER"
+    "LC_TELEPHONE"
+    "LC_TIME"
+  ];
+in {
+  options = {
+    nixos.system.locale = {
+      enable = mkOption {
+        type = lib.types.bool;
+        default = true;
+        description = "Enable locale configuration.";
+      };
+      timeZone = mkOption {
+        type = lib.types.str;
+        default = null;
+        description = "The system time zone (e.g., \"Europe/Stockholm\").";
+      };
+      defaultLocale = mkOption {
+        type = lib.types.str;
+        default = null;
+        description = "The default locale for the system (e.g., \"en_US.UTF-8\").";
+      };
+      extraLocale = mkOption {
+        type = lib.types.str;
+        default = null;
+        description = ''
+          The locale to use for specific LC_* categories.
+          If set, it will override the categories specified in `locale.categories`.
+          Example: "sv_SE.UTF-8".
+        '';
+      };
+      categories = mkOption {
+        type = lib.types.listOf lib.types.str;
+        default = defaultCategories;
+        description = ''
+          List of LC_* categories to override with `locale.extraLocale`.
+          Defaults to common locale categories.
+        '';
+      };
+    };
+  };
+
+  config = mkIf cfg.enable {
+    time.timeZone = mkDefault cfg.timeZone;
+    i18n.defaultLocale = mkDefault cfg.defaultLocale;
+    i18n.extraLocaleSettings = mkIf (cfg.extraLocale != null) (
+      lib.foldl' (attrs: lc: attrs // {"${lc}" = cfg.extraLocale;}) {} cfg.categories
+    );
+  };
+}