From: Arthur Alexander

Researchers at the World Bank have put together a variable called the Index of ethnolinguistic fractionalization. The index includes such measures as the probability that a resident of a country randomly encounters someone speaking another language. Out of
109 countries, Japan comes in second place in terms of homogeneity. S Korea is in first place and Japan is tied with Haiti and Portugal. The US ranks 61 out of 109, between Switzerland and Botswana.

The points already made are good ones; such discussions must be comparative to be meaningful: for example, comparing a country across different times, or with another country.