"Do you know this stranger?"
Translation:Znáte tohoto cizího člověka?
And IF your question is coming at this from a different angle, why assume that "Znáte tohoto cizince?" isn't accepted, just because it isn't the main translation shown above? We can't tell from your post exactly which perspective your question is coming from. Was an answer rejected, or are you just wondering? If a clear answer is wanted, asking a clear question would facilitate that.
And, if you were not aware of the Report button before nueby responded, it's important to use it, since it allows us to see exactly what a user's answer was, so we can tell you why it was not accepted.
Thanks for your cooperation.
Yes, my answer has been rejected; it's the reason why I asked. And, I didn't use report button, because I didn't now if my version of the sentence was right or not. I used the report button couple of time in the past but never got any response so I didn't know if my proposal was correct or not.
"Znáte tohoto cizince?" is accepted. But, since you didn't use the Report button for this one, we can't tell you why it was rejected. If a rejected translation is not obviously incorrect, we usually find a typo, missed word, or other small error when we can review a report. You can ask a question here, but please use the Report button even if you do, because it helps with troubleshooting.