TYPO3 Core

When installing new backend language packs via "Maintenance > Manage Language Packs" and changing the users language to a newly installed one does not translate the backend at all.

Due to wrong folder structure in "typo3conf/l10n/" the default language will be used.
The extensions' directory is wrongly nested (e.g. "de/backend/backend/"). The language pack .zip file already contains a directory but the new LanguagePackService appends the additional extension key directory to the extraction path.

Please compare changes in https://github.com/TYPO3/TYPO3.CMS/commit/780c30294b8604ed531adc0964962716fd454c46
From https://github.com/TYPO3/TYPO3.CMS/commit/780c30294b8604ed531adc0964962716fd454c46#diff-0ffae65169628463df81245ad35f51ebL140
To https://github.com/TYPO3/TYPO3.CMS/commit/780c30294b8604ed531adc0964962716fd454c46#diff-17130448f9806a2de97238ee80c8981fR236