it's hard to tell without knowing what kind of files are in a dlc (edata).
from a programmer's perpective, i think it might be because the us game can't handle the jp title and attachment names. since the game font (the picture of the characters, to be more precise) is limited to english letters only, it is possible that the game was modified to work only with maximum 127 characters (7-bit ASCII, standart american character encoding). when the game tries to read a japanese character, it will find a value that is over 128 (Shift-JIS first byte). such errors aren't handled so it simply crashes.
of course this is just a guess. i wonder, what is the exact moment when it crashes?
For some titles the moment you equip them the game freezes. For others they can be equipped but everything inside is null (funny thing is that the game itself writes null in the places where you should get something).
First off, thank you so much mamu for making all of this possible. Now I was wondering, is there supposed to be text underneath the area names when a new area is first introduced? Instead there's this big black box currently. Is this something in the eboot?