I first really learned about internationalization back in 2007-2008 from a Japanese theme user. He pretty much taught me what I needed to know to make my themes be worth using in other languages. Ever since then, I've made this a top priority. So, Yoichi, if you happen to find this post on WP Tavern, thanks!

A little note about the current theme on WP Tavern: it has a built-in Japanese stylesheet that auto-loads if the language is set to Japanese. :)