Oli_lar = Browse the categories THROUGH shoppingmalljapan - it does exactly what you wants. It keeps the Japanese intact, but also puts the English translation directly underneath all the text. Very very convenient.
However, it brings up two frames on the upper and lower parts of the screen, so I recommend right clicking the middle frame and choosing "open in new window" - that way, you'll lose all the unnecessary frames, and still have a fully translated YJ window.
You can also translate pages directly through
www.systranet.com - it requires a registration, but it is free.