A mistake in the sense that it ignores a standard that everyone else ignores, but delivers the correct language to 99.999% of the visitors?

(Hint: most people in Japan speak Japanese. There are 126,000,000 Japanese in Japan, compared to 1,000,000 foreigners. Most of the foreigners are Korean and Chinese and probably understand Japanese just fine. Hell, so do most Americans, probably. My point is: if you're in Japan, it's quite reasonable for a website to default to Japanese.)