I thought OS X was the same edition the world over (i.e the version of the OS X that came with my MBP when I brought it over in Australia for three months) runs in Japanese and English.
Is it just that the pamphlets and printed documents that come with the DVD are in English as opposed to Japanese.
Anyway, tempted to get Leopard when I'm out there late Nov/early Dec. I'm going to be there anyway... and 14800 Yen works out about 20-25 UK Pounds cheaper than the 85 Pounds it costs here in the UK. (Not sure if the Japanese price included the 5% Sales Tax... but even so, if you add that one, still about 20GBP less than the UK).