How to set up menu with links to pages in different languages on multilingual sites?
How about using "language neutral", or "Indépendant de langue" in French? That's an option you should have on the node "edit" form ("modifier" in French) for each of those 3 pages. And then combine this with creating a (single) menu link, in the appropriate menu of course.

That should make such link show up for both languages. That's how I always implement such pages, typically for pages similar to the ones you mentioned. Or sometimes in cases where some translation is still missing, and at least such page should be shown already in another language for which its content is available already.

FYI: I'm also using "determine language from the URL path").