I don't think that applies here. In fact, the key point is understanding the language. Let's say for example that I decided one day to learn Russian. I can now understand the language without the aid of an interpreter, but that does not mean that I wouldn't want to listen to the translation during any given activity to make sure that I fully understood.
In the present case, the fact that a person is able to understand what someone says in French without the aid of an interpreter shows that they understand the language. It is simply a way to determine whether they do.
As I said earlier, that does not mean that the person would never be allowed to use an interpreter in carrying out their duties. The idea is simply to define what “to understand” means.