What do translators do when they encounter this?

Say a book is going to be translated from English into French. This book contains a scene in which one character encounters someone who is speaking French to him and cannot understand them. The gist of that scene would be that he did not understand this person. If the book is being translated into French, the character would be speaking in French, right? So if this person is speaking French to him and he says he doesn’t understand, wouldn’t that be a little strange? What happens in a situation like this? Is the scene changed or kept the same?