"Whom" is properly correct in this case, but I'll concede that in colloquial English most people don't use it any more. I still do, though. Arguably, it's more important here than anywhere, as "kogo" literally means "whom", you actually can't use "kto" here like you could use "who" in English.
It does work in German. "Wen seht ihr?" German doesn't mix up "wer" and "wen". Even though most speakers of English don't use the word "whom," I personally like it when dealing with cases in foreign languages. Kogo in Polish and wen in German refer to who is being seen (accusative case/DO), not who is doing the seeing (nominative case/subject)..