[BUGFIX] Strict Standards - setLanguageKeys must be static

protected LocalizationUtility::setLanguageKeys() is only
called from static initializeLocalization and needs to
be static, too.

Resolves: #47983
Releases: 6.2
Change-Id: I7d0a4f5e8fc09ee049a6a7af1be65d5d5e8bf5e4
Reviewed-on: https://review.typo3.org/27506
Reviewed-by: Christian Kuhn
Tested-by: Christian Kuhn
Reviewed-by: Oliver Klee
Reviewed-by: Anja Leichsenring
Tested-by: Anja Leichsenring