I am owning the japanese GS-400 and it's menu language cannot be changed to English. In various forums I have tried to find out if anybody had successfully changed the language yet, but it seems that not, if yes, please let me know.
As it happened now and again that I had to look up the manual at last I created some small handy menu helpers in Excel which I laminated and now carry in my camera bag.
I don't have where to put it on the net for downloading, but if anybody is interested please let me know your email adress and I will send you those files.

I don't have any solutions, but this might be possible. People have modified the firmware in various makes of cameras, like Sony and JVC. If you google, and search by posts for make or model, "hack" or "modification", English, and "firmware" or "bios" and also search these forums (select search by post option if you get long threads).

The one problem that might be faced, if it is possible, is that the length and type of text for Japanese menus is different from English equivalent, even if it supports English text, it might store the text in a space of memory smaller then the English text can fit, needing the program code to be move, which might not be easily done. Their might be the possibility that the US firmware can be flashed to the Japanese version, but the camera might have differences that will make it not work, crash, or some things not work, or might even fry the camera.

But, there was a guy selling various black mamba models, he seems to have instructions in English, maybe he will know something as it would be in his interests to find a way for people to change to English menus.

(please note, I do not assert he is a good place to buy from, I don't know enough to say that).

http://www.supervideo.com/MXmouse.htm
http://www.supervideo.com/panason.htm
http://www.supervideo.com/GS400.htm
http://www.supervideo.com/MXExtreme.htm

Andreas's English menu aids can be found here. (http://media.pana3ccduser.com/wiki/index.php?Japanese%20to%20English%20Menu%20Aide)

...at last I created some small handy menu helpers in Excel which I laminated and now carry in my camera bag.
files.

I don't carry any menu helpers anymore.
All the important menu points are right there stuck to my CGA-DU21 battery packs.
Guy if there is interest I'll send you these spreadsheets so you may include them for download in your WIKI.

My question is: What is a Black Mamba as related to this thread???

black mamba is the nickname of the black body japanese model of the PV-GS400, see the photos of Waynes links