Personally I don't think ereaders will support czeck but perhaps others will prove me wrong. That said the best way to read electronic books, in my opinion, in her native language is to download and install open office software for free from
http://cs.openoffice.org/index.html. Once that is done open office will open any ebook files that you might download and will translate to her native language. Great things here is that it is fluent in czech it also speaks hundreds of other languages as well and can help her learn English because it can understand and work with both languages in the same document. Once you have the document in czech I believe you can then upload it to a reader and display it in czech as long as Open Office gets to the book to translate it to czech.
Do you also know about translate.google.com that also supports czech and can help with ay web site. Simply enter the web address of the web site into the translate box and the web site into what ever language you have told it to translate to.