Can a "maitre de conferences" be called "professor"? In the French system and other francophone systems, tenured faculty is divided in two categories: maitre de conferences, equivalent to associate professors if I understand correctly, and professeurs, equivalent to full professors.

I'm preparing a list of people for a meeting. I am required to distinguish between no title, Dr., and Prof. The question is simple: should I call the maitre de conferences "professors"? On the one hand, their title is not formally "professor". On the other hand, their rank is equivalent to "associate professor", and so they deserve to be called "professor".

I am interested in a general answer. Not necessarily only for my situation. Call this curiosity if you wish.
This is a very general answer, so for your case you may wish to wait for an answer specific to the French system.

My opinion is that you should never 'translate' someone else's title in this way. You said that a maitre de conference 'deserves' to be called professor. However, the title 'professor' is not a value judgement on a person, it's just a job title.

In the US, this job title is used quite liberally. Outside the US, the title is often restricted to the most senior staff. In the UK, for example, the job positions of lecturer and reader are equivalent to a US associate professor and full professorship respectively. So a maitre de conference wouldn't be equivalent to a professorship in the UK. If you call a maitre de conference a professor, will you also title British lecturers and readers 'professor'?

Many scientists work at institutions which are not universities, and hence, cannot be professors unless they have also have a university appointment. For example, even the director of a Max Planck Institute is not (automatically) a professor. Are academics employed in such institutions also to be upgraded to professor?

It would be strange to invent a professor title in any of these cases. At best, it will cause bemusement, at worst they will think that you haven't properly researched their background.