One other thing, I don't know what system menu your jap wii is on, but if it is less than 4.0 you could load this
Nintendo's Offical Wii BackUp Disk v1.31.rar
Use regionfrii to change it to jap. If it did load you can use it to install a savegame exploit, and format system.
It probably won't load, but worth a try.
As a last resource, I will try to fill the nand to use data management trick.
Wii Fit Jap swap is the same problem with Castlevania Judgment, Net Flix disk spin for 4 5s and stop spin >.<, i try to write the new disk but no luck too >.<
Nintendo's Offical Wii BackUp Disk v1.31.rar wo'nt load too :( i think it 4.1J or 4.3J. The fisrt wii i have been met with this solution >.<
Also, when trying discswap ensure that both disc have a jap disc header that are identical. Same region and disc id on the header.
Also, when doing the discswap with castlevania, start with castlevania disc, load the game to the title screen, press home on the wiimote and swap the disc, then press home on the wiimote again and go back to the game title menu. Very quickly as soon as you here the disc start to spin up, press home on the wiimote and reset on screen.
It is possible that like ssbb-pal, netflix disc also will not play with the jap language setting.