[TASK] Add field flag to LanguageMenuProcessor

This patch adds the missing field `flag` from the site configuration to
the LanguageMenuProcessor. The field could be used now to show the flag
icon in a language menu out of the box and without workarounds.

Resolves: #84926
Releases: master, 10.4
Change-Id: Ie27bed551a96cae8c6b20913105ff95db7f61942
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/65814
Tested-by: Simon Gilli <typo3@gilbertsoft.org>
Tested-by: TYPO3com <noreply@typo3.com>
Tested-by: Georg Ringer <georg.ringer@gmail.com>
Reviewed-by: Simon Gilli <typo3@gilbertsoft.org>
Reviewed-by: Georg Ringer <georg.ringer@gmail.com>

typo3/sysext/frontend/Classes/DataProcessing/LanguageMenuProcessor.php

@@ -202,6 +202,17 @@ class LanguageMenuProcessor implements DataProcessorInterface
                         'wrap' => ',"direction":|'
                     ]
                 ],
+                '25' => 'USER',
+                '25.' => [
+                    'userFunc' => 'TYPO3\CMS\Frontend\DataProcessing\LanguageMenuProcessor->getFieldAsJson',
+                    'language.' => [
+                        'data' => 'register:languageId'
+                    ],
+                    'field' => 'flag',
+                    'stdWrap.' => [
+                        'wrap' => ',"flag":|'
+                    ]
+                ],
                 '90' => 'TEXT',
                 '90.' => [
                     'value' => self::LINK_PLACEHOLDER,