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document.getElementById("restart").textContent = "Recommencer";
You need to state which Story Format you are using when you ask a question, as answers can be different for each one.
If you are using one of the built-in story fromats (Sugarcane, Jonah or Responsive) then you can use jQuery to modify the Restart link, do the following:
1. Add a StorySettings special passage to your story using the Story > Special Passages > StorySettings menu items.
2. Edit the StorySettings passage and check the Include the jQuery script library option.