You need to set the language in the options for German before you load your source file. If you already have the HTML0, then you need to change the laguage to German and resave it.
Also go to
https://www.mobileread.com/forums/showthread.php?t=11649 and download the Book Cleaner files to fix some gotchas in BD such as emdashes. For emdashes to work, when you make the LRF, set dashes to as-is.