My son also only speaks french and has been going to the clubs since he was 3, 5 years ago. Out of our 8 cruises, we’ve had French-speaking CM’s on about 4 cruises. Since the group activities are in English, even when they are available, the language barrier limits participation in those activities, but not always. Many factors come into play - interest of the kid, complexity of activities, etc. There is plenty to do outside of those anyway.
Kids also have a way of communicating with each other by other means if necessary. Depending on the period of the year, like during the March school break in Quebec, there might be plenty of other French-speaking kids. Because of that, the presence of French-speaking CM’s is not that important for us. It is mainly fun for them because they get to use French for the week and it reassures our son, but other CM’s will help him and call us if needed.
I would suggest going to the first open house on embarkation day and mention the issue to a few counselors. They’ll remember it for the rest of the week and usually will identify the CM’s that speak French if they are available.