Bonjour William,

Our grading system does actually take into account users' choice to not answer a question as evidence that they need to study/refresh the related lesson. That's why, at the start of every test, we actively ask users to skip questions on purpose when they don’t know the answer (rather than guess). Therefore, when you skip a question, the system simply assume you didn't know the answer, and so test you again later on.
So the worst that can happen is for you to be tested some more on your French, and as the saying goes: “C’est en forgeant que l’on devient forgeron”, i.e. “Practice makes perfect”.

Bonne journée !