Hello, guys, it's me again.
I realized that I have to patch file classes/PDF.php to support other encodings than the default iso-8859-1. Czech language needs cp1250 (or iso-8859-2) as we use a lot of strange characters. :-D Sadly, FPDF doesn't support UTF-8, so I made brand new cp1250 fonts and modified classes/PDF.php to use them when needed. I also attached czech translation.
You may review it at http://labs.logic.cz/patches/prestashop-0.9.7/invoices.patch. I hope it's now more generic solution.
The fonts can be downloaded here http://labs.logic.cz/patches/prestashop-0.9.7/fonts-cp1250.tar.gz.