That's right, Flemish..
But i've still never seen a Big poster outside a Belgium cinema..
I think they (Belgium 24"X15") are used in Holland and Luxembourg as well.
I have a theory...
Since customs and border control have been removed in Western Europe. It no longer matters what printer a film distributer use's. Why not get 'Thierry hates "French Grande"'s' printed in Belgium, if it is cheaper. Also Belgium is very close to 'Paris' compared with a lot of French City's.
I can see a situation where the Artist/Graphic Designer does the work, and the plates would be sent to Belgium for cheaper printing.
This still means they are French posters for use in France. They are never going to be used in Belgium, they don't have the Frames outside the cinema's to fit such a size of poster.
Although i haven't been to Belgium for over 10 years and wouldn't be surprised if they had converted to light boxes and 1-sheets.