I go there once a year, they have never spoken English when i was there. They are quite nice though. Pretty straightforward process, don't think you need to talk much, though they do ask a few questions about your state of health. I've always gone between 11 and 1, never seen a queue or another person there. You should go in the mornings though, and it's usually a few days before you can call and get your results -- it is not as fast as the site implies.
Also note that you need to not pee for at least 2 hours before going or the tests could be inaccurate. This is one of the things they will ask you, too.
I provide STD testing for men and women at my gynecological office.
The charges per test are:
HIV 440 Kč
Syphilis 350 Kč
Chlamydiae 350 Kč
Gonorrhea 350 Kč
Results are available immediately.
Dr K. Bittmanová, gynecologist, Mánesova 64, 120 00 Praha 2, tel. 603 551393, or 222 724592