@pHneutre, do you want all users who currently have “regular” as their title to get the title in French?
You can navigate to /admin/customize/site_texts and search for badges.regular.name. By editing that (for example, by adding a “1”), all titles will be updated to use the language in which you made the change. Then, you can revert the change, and all users who now have the “regular” title will have the French “Regular” title.
In general, I think this is a bug because changing the wording in one language shouldn’t update titles that use a different language. However, this could be an easy workaround for you to set all titles to French again.