I'm currently in Tokyo on business. As I travel around the city I've been spot checking Foursquare's coverage. To my surprise, coverage is excellent. Most popular eateries and nightspots have already been added. Many already have mayors. Foursquare specials abound.
Interestingly, Foursquare's UI has not been localized to Japanese. There is no Foursquare office in Japan and no substantial marketing efforts have been made. You might think that would significantly limit adoption but it hasn't. Tokyo is one of the most checked-in cities in the world. Some of the factors that are likely in play:
There is a lesson to be learned from all of this. Even if you don't have the budget to fully localize your app to locales like Japan and China, it pays to make sure you don't hinder motivated potential users from participating. We've observed Chinese users on Japanese web apps a number of times because they can happily add their Chinese UGC (user generated content.) Globalize your app even if you can't localize it. Use UTF8 for your character encoding everywhere. This will allow Japanese and Chinese users with a modicum of English to use your app and it will make it easier when you decide to enter the market with full force.