the guy at the game store has no idea. It can't low up your Wii based on the input. The input will make very little difference on the output. Especially since the input is actually exactly the same.
You'll probably find that the electronics inside the Japanese adapter and the EU adapter is actually exactly the same, and the only difference is the shape of the prongs on the outside (and the sticker on the front) since the multi-input AC adapters are pretty cheap to manufacture.
In other words, don't listen to the guy in the game store. You will not have any issues plugging a EU Adapter into a Japan Wii so long as the output voltages from both adapters are exactly the same.
EDIT: That employee was most likely trying to cover his arse in case something did happen. Personally, I would think you would be safer with the European adapter since it's actually designed for use with European power, which is what you are sourcing it from. You have to remember that the Japanese adapter has been designed for 100V 50/60Hz and yours being most likely 230v 50Hz.
Help! I'm stuck in a forum signature!
