The electricity on the plug is AC (alternating current). There are 2 "big" standards for frequency/voltage, and a truckload of different plugs/sockets (for more have a look at wikipedia). The electricity on your Wii is DC (direct current), for which frequency is irrelevant.
Now, just about any brick adapter will state what input it expects (japanese adapters state 100V~50/60Hz, as expected; a european one states 230V~50Hz, also as expected; these numbers may vary a bit, and some adapters take just about anything 100-240V~50/60Hz), and the output it produces, which for the Wii is 12V=3.7A.
What all this means is: as long as the output is the same (and it seems to be), it's perfectly safe to plug a european adapter to a japanese Wii. As I've already said, frequency is irrelevant for output, as it's DC (notice they use the = instead of the ~, = means it's "constant" as in DC, and ~ means it "waves" as in AC). So don't worry about that. Also this is totally unrelated to the framerate of your games, you don't need to worry about that either.
I say, don't worry (repeating myself), and just buy the european adaptor. It's more power efficient and everything! And if anything gets messed up, it's because your Wii is already in serious trouble to begin with, and you need to get it fixed.
Just make sure whatever adapter you do buy (if you don't buy Nintendo's adapter, for instance) has exactly the same output as your current Wii adaptor (12V=3.7A).
