Danger: informational wall of text *WILL* crit you for 18570 hit points.
As was noted above the files that must be replaced from the japanese version are:
voice.dat
voice.dat.tbl
msgvo.dat
msgvo.dat.tbl
I initially tried to do it using dslazy. I can now confirm that this solution may work on some cards and not on some others. I own an R4 with 1.18. What I have done is use dslazy, find the japanese rom and press unpack. When unpacking finished I renamed my NDS_UNPACK directory to japanese_NDS_UNPACK. Then I deleted the file "current.nds" from the dslazy folder. I then opened up the american rom, and clicked unpack. After this happened I navigated inside the japanese_NDS_UNPACK folder and into the "data" subdirectory I picked up the files above and copied them in the respective data directory inside NDS_UNPACK.
Afterwards I pressed pack and saved the new .nds file.
The result was a no$gba compatible rom but somehow did not work on some cards (firmware perhaps? can't tell really).
Then I fired up dsbuff. Google it, you ll find it. You ll come up with one complete .exe file you can use. This utility behaves *EXACTLY* like dslazy with one exception. It saves everything in the C:\dsbuff directory. If you follow the exact same procedure (1. open up jap rom, unpack it, rename the unpack directory, 2. delete current.nds, 3. open up american rom and unpack it, 4. copy files from jap to american unpack directory 5. pack rom) you should come up with something useful.
It works perfect on my R4, I will probably check if it works on my Supercard (nds lite slot) and report more. But the procedure must be the same for everyone, and I dont believe it will have any difference depending on the program you use.
Can anyone who has been succesful be kind enough to post their card combo? If you failed, can you give more details where did you encounter the problem? because the solution is like piss-easy to perform and I believe these instructions are more or less very simple...
Thanx in advance to all the posters above.
P.S. I believe dsbuff may require the .NET runtime to run, I can't tell for sure since I have all .NETs in my machine, but, meh, who knows.
P.S. (2) so far I have played enough to reach the point where the gatekeeper opens up your portal to Vyers castle. Everything looks cool.
P.S. (3) The reasoning behind undubing this is that the japanese voice actors in general are FAR MORE PROFESSIONAL than their american counterparts, people there actually study voice acting :-) And after all, this is an anime-like game, you expect to listen to japanese voice acting, right? ;-) Plus the voice actors for disgaea are quite legendary in the field, the main character being voiced by the girl (!!!) who does Ogioue in Genshigen anime series, among other credits.
P.S (4) soz for da long post, but...