I'm noticing that the Japanese Dungeon.DAT is exactly the same as the English one aside from some extra zeroes (probably to insert longer strings).
It will likely be possible to just write a program to convert each file if they all follow the same pattern.
I'll manually finish the Dungeon.dat and then use the program on the original Japanese dungeon.dat and see if there's any difference.
As for Hex Editors, MadEdit looks and runs a lot better than WindHex, has built-in S-JIS support, and is open source.
Tobster7: So far they are the same (I know Japanese, so that helps).
Beside my files i tried to find the data where the menu texts are stored. In the USA Version this text is outsourced in "InProgramTxt.dat". I know that the word "BMG" (Background music) is written in normal letters in the JAP version. So if anybody finds Kanji and the single word "BGM" inside then we have found the menu texts. Please look for this pattern if you edit your files.