You can buy the Japanese version of "Wii Fit" from Play Asia or off e-bay,
But it will only work if you have installed a "modchip" in your Wii or have a Japanese console because the format of the Japanese versions of the games is different.
There is also a new device called the "Wii Freeloader" which claims to enable you to play ALL formats without installing a "modchip". It’s is a "softmod" device, a disk you pop in, and when it stops, you eject it, load your game and play your game.
Even then though, you won’t be able to read it because all the words are written in Japanese!
However, the European version of Wii Fit is due to be released in April, while the United States and Canadian versions are due to be released in May. I’m really looking forward to it myself, think I’ll wait.