As someone who has shot in Czech from 1998 to 2008 I can tell you the answer.
Only cast girls and guys who can speak good English. All other options will cost sales.
The girls in Czech now tend to speak better English that 10 years ago. So maybe you can find some good ones.
Dubbing well is expensive, sub title on a porn film.
Native speaking.
All compromises.
I have to ask why shoot it in Eastern Europe. Why not the UK? Thats part of Europe. Is it a cost or beauty issue?