If you go to see the full results: http://www.imo-official.org/results.aspx , you can see that asian country have a lot of good results. Do you know what the cause is?

Asia has a culture of celebrating (and desiring) academic achievement far more than Western culture does. In the West, schools gets blamed for poor results. In the East, poor results are treated as far more of a reflection on the family and on the parents.

Statements like "I am not very good at maths" seem to be culturally acceptable in the West, whereas you might as well be saying "I have an STD" in the East.

Source: I went to school in England, in the US and in India and so have seen these differences first hand.

Probably for the complexity and irregularity of the number system their languages use.

http://www.sf.airnet.ne.jp/ts/language/number.html

Asian language tend to use almost regular base of 10 number systems.

Other languages, like English, has historical debt on the language system.

French is especially bad.

There is a study that shows that European languages can make number acquisition and arithmetic more difficult and I noticed that sometime it's easier for my daughter (5 year old) to do addition and subtraction in Khmer than in French.

But I don't see how it would cause a problem long term for the best student. I believe that STEM is still esteemed in Asia and less so in Europe.

Surely the advanced math problems featured in the Olympiad are not based on simple arithmetic?

It has nothing to do with language and everything to do with parents.

Difference in cultural expectations? If these sorts of skills are deemed important (for a significant period of history) I don't see why the nurture couldn't affect the nature in the long run though. I wonder what the math talents of Asian children adopted into families with different parental expectations are like, on average.