What's the current BIOS version? Is it older than the one available at http://www-307.ibm.com/pc/support/ [...] CKQL2.html?

I thought that file pjjt41a.exe was not Japanese, but that file pjj241a.exe is Japanese. So they provide 2 different files, but both are Japanese?

I don't understand where your issue is, but I don't have an old PC to test it. When I double-click pjjt41a.exe in Windows XP, the DOS window that opens up is in English, not in Japanese. By following on-screen instructions, a bootable diskette is created. It could then be used to boot the PC and flash the BIOS.

When I executed pjjt41a.exe , the following occurred:

1. The diskette was formatted
2. The program then displayed "Extraction complete"

Here's an excerpt from the README.1ST on the diskette:

* SHELL=FLASH2.EXE option1 option2 option3......optionxx *
* *
* OPTIONS *
* ------- *
* *
* /U UNATTENDED MODE. ALL DEFAULTS ARE ASSUMED (UNLESS *
* OVERRIDDEN BY OTHER OPTION STATEMENTS) AND ALL *
* PROMPTS ARE BYPASSED. ANY ERRORS WILL RESULT IN *
* PROGRAM TERMINATION WITHOUT ANY ERROR INDICATION. *
* <<< USE THIS OPTION WITH CAUTION >>> *
* *
* /US COUNTRY CODE (LANGUAGE). THIS CAN BE ANY OF THE *
* VALID COUNTRY CODES (UK, SV, SU, SP, PO, NO, NL, *
* IT, GR, FR, DK, CF, CE, BE, LA, US, SF, SG, BR,JP, *

As you can see, that isn't Japanese. The Autoexec.bat starts flash2.exe with no options.

BTW, the BIOS file for US English is $IMAGEPJ.USF.