If you have dumps of those firmwares and a way to safely replace them then you might be able to change the text by opening the files in a hex editor, searching for the place where the text is stored and writing new text into it, but make sure to be in overwrite mode and not write anything longer than the existing text. I'm not sure if that Japanese text is encoded as standard Unicode so you might have more trouble finding that in the Japanese firmware, and I'm not sure if this can work at all, it's just my thought because text can often be replaced like this.