|What are the problems you are experiencing ?|
You should be able to use other languages in the books, but the filenames must be in ASCII for now.
Also, remember to use UTF-8 encoding in the XHTML files and specify this in their headers.
For instance, start them with:
<?xml version="1.0" encoding="utf-8"?>
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.1//EN"
<meta http-equiv="Content-Type" content="text/html; charset=utf-8" />