[TASK] Rename "Extensions" backend module

Renames backend module Extension manager to "Extensions".
Also lowers language barriere in "Manage Extensions".

Resolves: #55236
Releases: master
Change-Id: Id8ca9545128807adcc925f3e00f705febae93d2b
Reviewed-on: http://review.typo3.org/33381
Reviewed-by: Georg Ringer <georg.ringer@gmail.com>
Tested-by: Georg Ringer <georg.ringer@gmail.com>
Reviewed-by: Ingo Pfennigstorf <i.pfennigstorf@gmail.com>
Tested-by: Ingo Pfennigstorf <i.pfennigstorf@gmail.com>
Reviewed-by: Mathias Brodala <mbrodala@pagemachine.de>
Reviewed-by: Xavier Perseguers <xavier@typo3.org>
Tested-by: Xavier Perseguers <xavier@typo3.org>
Reviewed-by: Markus Klein <klein.t3@reelworx.at>
Tested-by: Markus Klein <klein.t3@reelworx.at>