nixos/i18n: warn when the user has a badly-configured supportedLocales

  ###### implementation

  config = {

    warnings =

      lib.optional

        (

          (lib.subtractLists config.i18n.supportedLocales (

            builtins.map

              (l: (lib.replaceStrings [ "utf8" "utf-8" "UTF8" ] [ "UTF-8" "UTF-8" "UTF-8" ] l) + "/UTF-8")

              (

                [ config.i18n.defaultLocale ]

                ++ config.i18n.extraLocales

                ++ (lib.attrValues (lib.filterAttrs (n: v: n != "LANGUAGE") config.i18n.extraLocaleSettings))

              )

          )) != [ ]

        )

        ''

          `i18n.supportedLocales` is deprecated in favor of `i18n.extraLocales`,

          and it seems you are using `i18n.supportedLocales` and forgot to

          include some locales specified in `i18n.defaultLocale`,

          `i18n.extraLocales` or `i18n.extraLocaleSettings`.

          If you're trying to install additional locales not specified in

          `i18n.defaultLocale` or `i18n.extraLocaleSettings`, consider adding

          only those locales to `i18n.extraLocales`.

        '';

    environment.systemPackages =

      # We increase the priority a little, so that plain glibc in systemPackages can't win.