Added some more translations that I stumbled over. Also added my name to it.
Please note however, the translation in line 3120 (book will be deleted from database) and the next translation (from hard disk) don't work well that way in german. Please consider defining two separate translations (one with and one without the hard disk). The german translation grammatically works the way I committed it here, but it's really not how one would write it if they had a choice. The "gelöscht" should always be at the end in german.
Updated German translations in messages.po, correcting various phrases and ensuring consistency. But most importantly added missing translations. Didn't check all translations though, just those that catched my attention
Sebastian Holzer