[BUGFIX] Improvement of LanguageCacheClearer

The cache is cleared no matter if the call is valid
or not. As a solution the relevant line needs to be moved
inside the if clause

Change-Id: I171b28786094a75f9e971685f4384d6631ab9c52
Fixes: #47912
Releases: 6.2,6.1,6.0,4.7
Reviewed-on: https://review.typo3.org/20489
Reviewed-by: Philipp Gampe
Tested-by: Philipp Gampe
Reviewed-by: Stefan Neufeind
Tested-by: Stefan Neufeind