I thought there was more to it than just setting the locale. I've already flashed it so I can't really check now, but I thought there were a couple other tweaks done, unless I'm thinking of Cacko.
I'm now also curious about Conics' option to provide provide an "English setup and retain Japanese" for an additional fee.
If you have the Sharp ROM the difference between Japanese and English is a simple setting in locale.conf
I don't want to put down the Conics product because I know nothing about it. If you only adjust the locale, there are many things you can do to make it a bit nicer:
You can add English help files that don't come with the ROM by default and you can remove the input methods for Japanese. For example, when you are adding a postal code in an address in the QT address book automatically a special number input method pops up but it's designed for Japanese postal codes.
There are a couple of translation files that are missing I think, media player perhaps ?
Finally, the categories database has hard coded "business" and "personal" options but written in katakana. It's possible they modified the ROM with recompiled apps so those are removed.