I can explain this! In Korean (also in many other languages) a question can often be structure the same way the answer is with minor changes (~ㅂ니다 -> ~ㅂ니까 in this case). In English the answer to When are people happy? is of the format People are happy when.... Thus, the question in Korean can be constructed with the rearranged English question People are happy when? in mind. 사람들 is the topic, 행복하다 is the verb and 언제 is an adverb. Since Korean syntax for a sentence is usually (topic) (adverb) (verb), we get the question 사림들은 언제 행복합니까?.