How are multilingual scenes handled when the story's main language is translated into that of its minor one?

Question

Say you have an English book in which there is a particular dialogue where one of a handful of participating characters is speaking French. In this dialogue the fact that he's speaking French is a central point of the scene, as none of the other characters can understand him, which is necessary for the scene to make any sense.What if this book were translated into French, thus homogenizing the scene's linguistics and vanishing the source of internal confusion, thereby creating authentic confusion on the reader's part?

Answer 1

“Buenos días,” he said, “soy el asesinó que andas buscando.” I’m the murderer your looking for, he added.

The detective shook his head. “I don’t understand what you’re saying!”

OR

He entered and told the detective in Spanish that he was murderer he had been looking for. The detective shook his head. “I don’t understand what you’re saying!”