modules: system: language: make it configurable

modules/system/language.nix

@@ -1,7 +1,22 @@
 # Language settings
-{ ... }:
-
+{ config, lib, ... }:
+let
+  cfg = config.my.system.language;
+in
 {
-  # Select internationalisation properties.
-  i18n.defaultLocale = "en_US.UTF-8";
+  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;
+  };
 }