Well, for what it is worth I've just been on a 16 day vacation in Tokyo and I can assure you that at least 90% don't speak english at all. They will get OK, yes, no and Coca-Cola. But that's all.
And oh, around half that's 50% : ) were using an iPhone.
Exactly. I've been living here for almost 15 years, and the vast majority of people can't even hold a basic conversation in English.
If your app isn't localized in Japanese, 99% of Japanese users won't even look at the app's description on the App Store. When you know that in Japan, iPhone is the top-selling phone - not just the top selling smartphone, not localizing your app results in ignoring a large part of the market.
Of course, as an app developer you have to ask yourself if it's worth the cost, but it's an outdated idea to think that you don't need to localize your content for being successful in foreign markets.