[BUGFIX] Allow user defined locales to work with TSFE->sL()

TYPO3 uses a list of "known" languages defined in
TYPO3\CMS\Core\Localization\Locales.

This list can be extended by the user, but these changes
are not taken into account when using
$GLOBALS['TSFE']->sL() to fetch the translation.

Make TypoScriptFrontendController call the needed method
"initialize()" to make it work.

Resolves: #53973
Releases: 6.2, 6.1
Change-Id: I9fc80ba976bf796f272591ac741d8bf4e92cbeb3
Reviewed-on: https://review.typo3.org/25685
Reviewed-by: Wouter Wolters
Reviewed-by: Georg Ringer
Tested-by: Georg Ringer