Translation:Who smokes more, the Czechs or the Slovaks?
Should it not be "smoke" instead of "smokes"? "Czechs" and "Slovaks" are plural forms.
It seems that many grammar authors tend to avoid this question; I could not find a significant reference for it. But I have hardlly ever read or heard a question starting with “who are….” , “who smoke…” etc. Of course when “who” is used as a relative (rather than interrogatory) pronoun it can be followed by the plural (“Those who smoke…”) but that is a different topic.
I would agree, if "who" were a relative pronoun here, as in "the people who smoke a lot ...".
But it is an interrogative pronoun (question word) here. And this goes with the third person singular, because usually you don't know if the answer will be singular or plural. The second part of the sentence is not mandatory. It could well only be: "Who smokes more?". You would not say "smoke" here.