Tada.....
Following the topic:
https://www.mobileread.com/forums/showthread.php?t=58901 I just installed fonts on my SD card
Small adjustment inside TEST_defaultnewfonts.epub:
- replace ->
/abook/ to "/media/mmcblk0p1/" in the template.css
- edit the defaultnewfonts.xhtml (add some test line with polish characters plus convert from pure ANSI to UTF-8)
and I have polish characters on my iRex DR800S with the standard ADE reader. First step done.
Sorry for a little bit of topic discussion.