I strongly suggest using calibre to manage your ebooks. Sony's software is notoriously buggy and feature-deficient; its only real value is for buying books from the Sony store (if for some reason you should want to do that), and even then, once you've bought them, calibre will take care of them for you. Calibre support and download information can be found a few boards down the way, right here on MobileRead.
Once you've cleaned off the German books, stop by the ebook section of MobileRead to pick up a good selection of ebooks to load on it; they're both gratis and libre, not to mention some of the best-formatted ebooks available, since they were built by dedicated MobileRead members. Project Gutenberg, ManyBooks, Feedbooks, and various other sites can supply you with tens of thousands more.