i18n: Add charset related settings

  - `i18n.extraLocales` should now be the preferred way to install additional locales.

  - `i18n.supportedLocales` is now considered an implementation detail and will be hidden from the documentation. But the option will still continue to work.

  - `i18n.supportedLocales` will now trigger a warning when it omits any locale set in `i18n.defaultLocale`, `i18n.extraLocales` or `i18n.extraLocaleSettings`.

  - The options `i18n.defaultCharset` & `i18n.localeCharsets` were added, and they complement `i18n.defaultLocale` & `i18n.extraLocaleSettings` respectively - allowing to control the character set used per locale setting.

- `titaniumenv`, `titanium`, and `titanium-alloy` have been removed due to lack of maintenance in Nixpkgs []{#sec-nixpkgs-release-25.05-incompatibilities-titanium-removed}.

  ...

}:

let

  sanitizeUTF8Capitalization =

    lang: (lib.replaceStrings [ "utf8" "utf-8" "UTF8" ] [ "UTF-8" "UTF-8" "UTF-8" ] lang);

  aggregatedLocales =

    (builtins.map

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

      (

        [ config.i18n.defaultLocale ]

        ++ (lib.optionals (builtins.isList config.i18n.extraLocales) config.i18n.extraLocales)

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

      )

    )

    [

      "${config.i18n.defaultLocale}/${config.i18n.defaultCharset}"

    ]

    ++ lib.pipe config.i18n.extraLocaleSettings [

      # TODO: Explain why is this filter added here...

      (lib.filterAttrs (n: v: n != "LANGUAGE"))

      (lib.mapAttrs (n: v: (sanitizeUTF8Capitalization v)))

      (lib.mapAttrsToList (LCRole: lang: lang + "/" + (config.i18n.localeCharsets.${LCRole} or "UTF-8")))

    ]

    ++ (builtins.map sanitizeUTF8Capitalization (

      lib.optionals (builtins.isList config.i18n.extraLocales) config.i18n.extraLocales

    ))

    ++ (lib.optional (builtins.isString config.i18n.extraLocales) config.i18n.extraLocales);

in

{

        default = "en_US.UTF-8";

        example = "nl_NL.UTF-8";

        description = ''

          The default locale.  It determines the language for program

          messages, the format for dates and times, sort order, and so on.

          It also determines the character set, such as UTF-8.

          The default locale. It determines the language for program messages,

          the format for dates and times, sort order, and so on. Setting the

          default character set is done via {option}`i18n.defaultCharset`.

        '';

      };

      defaultCharset = lib.mkOption {

        type = lib.types.str;

        default = "UTF-8";

        example = "ISO-8859-8";

        description = ''

          The default locale character set.

        '';

      };

      extraLocales = lib.mkOption {

        type = lib.types.either (lib.types.listOf lib.types.str) (lib.types.enum [ "all" ]);

        default = [ ];

        example = [ "nl_NL.UTF-8" ];

        example = [ "nl_NL.UTF-8/UTF-8" ];

        description = ''

          Additional locales that the system should support, besides the ones

          configured with {option}`i18n.defaultLocale` and

          LC_TIME = "de_DE.UTF-8";

        };

        description = ''

          A set of additional system-wide locale settings other than

          `LANG` which can be configured with

          {option}`i18n.defaultLocale`.

          A set of additional system-wide locale settings other than `LANG`

          which can be configured with {option}`i18n.defaultLocale`. Note that

          the `/UTF-8` suffix used in {option}`i18n.extraLocales` indicates a

          character set, and it must not be added manually here. To use a

          non-`UTF-8` character set such as ISO-XXXX-8, the

          {option}`i18n.localeCharsets` can be used.

        '';

      };

      localeCharsets = lib.mkOption {

        type = lib.types.attrsOf lib.types.str;

        default = { };

        example = {

          LC_MESSAGES = "ISO-8859-15";

          LC_TIME = "ISO-8859-1";

        };

        description = ''

          Per each {option}`i18n.extraLocaleSettings`, choose the character set

          to use for it. Essentially defaults to UTF-8 for all of them.

        '';

      };