Since I had the problem right now I thougt a little advice for Win 10 would be the right thing to do. Didn't find anything about that using the Search-option.
To set the System Locale in Win 10 you have to follow these steps:
1. Start -> Settings
2. Open: "Time and Language"
3. on Date and Time-Tab under related settings -> "additional Date, Time and regional Settings"
4. additional window will open, there choose tab "Administration" there "change system locale"
5. Select Japanese and klick OK
Sorry for my bad english, if I made a mistake feel free to correct me.