Ah, you're a Frenchman, considering you say you're from where triathlon originated?
https://en.wikipedia.org/wiki/Triathlon#History
The Netherlands doesn't really have a national sport. Some candidates and why they may or may not be a national sport:
- Football; like almost anywhere in the world it's by far the most popular sport in terms of viewing numbers, fans, and so on - however, one can hardly call the world's most global sport any country's 'national sport'
- Korfball; obviously the Netherlands has always dominated this to kabaddi-like levels for India, but in reality not many people actually care; the national competition's final is usually live on TV (paid channel nowadays), but that's about it, you won't really see the world championships on TV for example, perhaps a very short bit of highlights of the final.
- Speed skating; by far our most succesful Olympic sport and generally pretty popular (though nowhere near as close as football); usually divided between two sides, one with people who like it and one with people who say it's just for old men; it's on TV pretty often in the winter, but nowhere near as close as the amount of time it had 15 years ago, World Cups are generally not shown live (they are on the public broadcaster's website, not on TV); quite a lot of people own a pair of skates and you'll see a lot of people on the lakes in the rare case they're all frozen, so it might have a chance for being the national sport
- Kaatsen/'Frisian handball'; weird sport practiced in a weird province of the country of which I have no idea how it works and how popular it is over there in reality, rest of the country doesn't give two craps; https://en.wikipedia.org/wiki/Frisian_handball