The only difference (aside from the language stuff and the fact that the North American boxart images are used) is that the Play on Wii boxes feature a curved top border in the blue area. If we're lucky, the covers will be reversible like the Japanese ones. We probably won't be that lucky!
We have yet to see the box designs for the European NEW PLAY CONTROL! series. We'll keep you updated, because we love pointing out that the European title for the series seems to officially be NEW PLAY CONTROL! in all caps.
Source: Mario Power Tennis