[FEATURE] Add Gaelic (gd) as supported language

This change adds support for Scottish Gaelic to be selected
as TYPO3 Language and to connect to TYPO3 Localizations
(XLF files, Site Configuration) by default.

Scottish Gaelic language is spoken in Scotland.

[1] https://en.wikipedia.org/wiki/Scottish_Gaelic
[2] https://en.wikipedia.org/wiki/List_of_ISO_639_language_codes

Resolves: #103255
Releases: main
Change-Id: I182ca93c38dc3a9d652e63d1c576cb37f7b36e3e
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/83318


Reviewed-by: Frank Nägler <frank.naegler@typo3.com>
Tested-by: Frank Nägler <frank.naegler@typo3.com>
Tested-by: core-ci <typo3@b13.com>
Reviewed-by: Stefan Bürk <stefan@buerk.tech>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: Stefan Bürk <stefan@buerk.tech>

typo3/sysext/core/Classes/Localization/Locales.php

@@ -61,6 +61,7 @@ class Locales implements SingletonInterface
         'fo' => 'Faroese',
         'fr' => 'French',
         'fr_CA' => 'French (Canada)',
+        'gd' => 'Scottish Gaelic',
         'gl' => 'Galician',
         'he' => 'Hebrew',
         'hi' => 'Hindi',

typo3/sysext/core/Documentation/Changelog/13.1/Feature-103255-NativeSupportForLanguageGaelicAdded.rst

+.. include:: /Includes.rst.txt
+
+.. _feature-103255:
+
+====================================================================
+Feature: #103255 - Native support for language Scottish Gaelic added
+====================================================================
+
+See :issue:`103255`
+
+Description
+===========
+
+TYPO3 now supports Scottish Gaelic. Scottish Gaelic language is spoken in Scotland.
+
+The ISO 639-1 code for Scottish Gaelic is "gd", which is how TYPO3
+accesses the language internally.
+
+
+Impact
+======
+
+It is now possible to
+
+*  Fetch translated labels from translations.typo3.org / CrowdIn automatically
+   within the TYPO3 Backend
+*  Switch the backend interface to Scottish Gaelic language
+*  Create a new language in a site configuration using Scottish Gaelic
+*  Create translation files with the "gd" prefix (such as `gd.locallang.xlf`)
+   to create your own labels
+
+and TYPO3 will pick Scottish Gaelic as a language just like any other supported language.
+
+.. index:: Backend, Frontend, ext:core