[BUGFIX] Labels for grouping bookmarks are missing

Using function sL to load locallang with labels for bookmarks

Fixes: #44691
Releases: 6.1, 6.0
Change-Id: I509aa06e379e1397fea50c6a2c137db3994eff2a
Reviewed-on: https://review.typo3.org/17955
Reviewed-by: Wouter Wolters
Tested-by: Wouter Wolters
Reviewed-by: Christian Kuhn
Tested-by: Christian Kuhn