nixos: create 'modules/nixos' folder

Let's consolidate all modules under one path, so that NixOS,
home-manager, and nix-darwin (if I ever end up using it down the line)
would go under the same folder.

modules/nixos/system/language/default.nix

@@ -0,0 +1,22 @@
+# Language settings
+{ config, lib, ... }:
+let
+  cfg = config.my.system.language;
+in
+{
+  options.my.system.language = with lib; {
+    enable = my.mkDisableOption "language configuration";
+
+    locale = mkOption {
+      type = types.str;
+      default = "en_US.UTF-8";
+      example = "fr_FR.UTF-8";
+      description = "Which locale to use for the system";
+    };
+  };
+
+  config = lib.mkIf cfg.enable {
+    # Select internationalisation properties.
+    i18n.defaultLocale = cfg.locale;
+  };
+}