in TYPO3 v9.5 user defined locales can not be added by an extension any more
At least in TYPO3 v. 7.x ist was possible for an extension to add a new local definition to the the TYPO3 localisation system.
This was posible by adding
$GLOBALS['TYPO3_CONF_VARS']['SYS']['localization']['locales']['user']['en'] = 'English';
ext_localconf.php of an extension.
In TYPO3 v9.5 this does not work any more because the method
typo3/sysext/core/Classes/Localization/Locales.php::initialize() which inizaliazes all the locales,
is called from
typo3/sysext/core/Classes/Core/Bootstrap.php in line 123 but the
ext_localconf.php are included after that in line 138 of the