I think I messed up my Wii even more while searching through the forums.
I tried to replace my system settings with the System Settings Replace Mod and couldn't find a Japanese setting so I used an English one. After doing this I was able to install the wads that wouldn't install earlier (IOS50-64-v4889.wad and SystemMenu-NTSC-J-v384.wad)
After installing these wads I restarted my Wii and Bootmii still works but now I cannot load Preloader anymore. In bootmii's info it says I have a 3.4J wii now but when I try to load the System Menu it's a blank screen again.
Would really appreciate some advice.