I have a US Wii U and a US 3DS, gave my Mii a Japanese name without hacking both.(Not even the Wii mode).
All you need is a DS version of Tomodachi Collection(Legitimate copy or flashcart doesn't matter,but if you have a flashcart, you can run the fan translated version).
1. Run that game on whatever console in the DS family, play until you can transfer Miis from the Wii.
2. Create a Mii with the exact Japanese name you want.
3. Choose the "Transfer to Mii channel"option on the DS game, and launch Mii channel on Wii mode.
4. Press A+B+1+2 to unlock communication w/ Nintendo DS if you haven't yet, and transfer the Mii from DS to Wii.
5.The Mii will appear in blue pants(uneditable), but it does not matter. Go back to Wii U mode and run Mii maker, choose to copy all Miis from the Wii mode.
6. You should have that Japanses Mii ready, and it should be editable.
Hope this helps