Update de.yml

platform:  # supported: 2.2.4, 2.3.2
backlogs:  # supported: 1.0.5
ruby:  # supported: 1.9.3, 2.0.0

Please merge this minor update for German translation.
The remaining 3 labels field_tracker_id. label_master_backlog, label_release_plural listed on http://www.redminebacklogs.net/en/translations.html are OK and should not be translated.