That's a fairly mean thing to do to an American reader; they should have assumed that the majority of their readers would not understand it. They might as well have sold the novel in the original German for all the good it did them. Mann, of course, assumed that his readers would know French and so the original German edition doesn't translate the French.

Clawdia teases him about his reticence and says that he should have approached her earlier and that it is too late because she is leaving the next day. She feels sorry for Joachim because she says he's going to die. She laughs at conventional morality and then he tells her the story of Hippe and the pencil, then praises her and she calls him a profoundly German suitor.

For what it's worth, that's the only extended part of the novel in French.