Yes you can change the fonts, we just do not know how at the moment ;-).
Do not worry. Our resident hackers have alredy something done, they just do not want to release it, because at the moment there is no way to unbrick the device if something goes wrong.
You can still read Czech (or ANY language of your choice) books on the device.
Even without updating fonts.
Just create a pdf file with page size 90mm x 115mm
and do not forget to embed fonts into the document.
You can download Sony Library software and see what will your document look like on a Reader.
You can create pdf files using OpenOffice.org