This is a sample design question for the position of Frontend Engineer at Amazon

If you were to design a web app that uses flash cards to help people learn a foreign language. The flashcard front side shows a word in the first language of the user, and when being hovered/clicked on, the flash card will flip and show the word in a foreign language. How would you go about designing this? Please talk from 3 aspects: server-client communication(including APIs), UI/UX design, optimization for performance.