I don’t understand why there is a difference, but if you go ahead and install in English you should be able to change the language by going to Miscellaneous, then settings then language and choosing Deutsch.
You can download an archive with the latest German translation files from the link in my signature and extract them into the directory \Translations. After a restart of the GUI, the German language should be selectable.
But it’s pretty odd, since at least the Russian and Chinese translations (the ones done by COMODO), should be delivered with the updater, in case they are missing. Therefore, if they were missing, the updater should notify you, that there are updates available and at least install these additional languages.