I guess I didn't think people would do that. Good point. Anyhow, I still stick to my opinion about the focus of the course. If I wanted my English grammar picked at and corrected, I would take an English course. Instead, I am trying to learn Czech. Czech is hard enough to comprehend on its own. Also considering that many people who take this course are not native English speakers, it could be nice to get a correction/warning in case of English grammar error instead of marking the whole answer wrong. I understood the meaning of the Czech sentence and translated it correctly. That was the point of the question. The part where I wrote smoke or smokes is really so irrelevant to my learning of Czech nations and the use of Czech word 'procent'.
As a native English speaker, there are times when I would construe "percentage" as singular. For example, "Is the percentage of Germans who smoke higher than the percentage of Frenchmen?"
On the other hand, in the sentence here I would say "percent" rather than "percentage". After all, it is not the "percentage" that smokes, for the percentage is only an abstract number. What you are really asking here is "How many per hundred (percent) of Germans smoke."
As others have said, native/near-native English speakers would not say "how many percent/percentage" but would use "what percent/percentage instead." Since I had the word bank up for this, I couldn't get it wrong... but that would be very easy to do using the keyboard for translation.
I understand the point of learning the Czech phrasing, and appreciate knowing it now. But the English sentence is just wrong..