In my country, to be honest, the level of diversity is high in terms of culture, but law in terms of language. I think there are two factors why the society of Japan is strange.
First, Japanese people are tend to accept foreign cultures. Traditionally, they have been affected by Chinese culture because Japan is one of the east Asia countries which China brought about a big impact. After the modern era, however, they have accepted European or American culture. This is because they considered that they had to come up these countries in terms of political systems, economic systems, and culture. So people in Japan have many cultural traditions from China, nations in Western Europe, and US.
Second, (I don’t have time to write this now. So I will try this tomorrow )