We don't have an official national sport (thankfully), but quite a comprehensive list of traditional sports/games recognised as such
They all are regional/local games, Some of them will be played in one city only and a good chunk of them should be considered dead and to be museum material. I'd consider most of them to be mere 'games' and not really 'sports' as we think of them. Sportswise, the most interesting ones are the descendants from the ancient paume game (ballon au poing, balle au tambourin, longue paume...) which are very similar to belgian balle pelote or allready mentioned frisian kaatsen.
If you had to pick one as 'the national sport' though that would be pétanque, as it's the only one (with pelote basque and boule lyonnaise) with a national reach.