b2evo currently only transliterates some German and French umlauts in URL titles. That means it doesn't know about the Czech letters and strips them -- I have an idea how to make b2evo transliterate more chars, but this feature won't be available any time soon.

A workaround is to manually make the URL title look nice by editing it before publishing a post. Remember you only can use ASCII chars in URL titles.

Use this plugin http://forums.b2evolution.net/viewtopic.php?t=15416

Just add your own patterns to transliterate special Czech letters.

Thank you all!