It was petitioned at NA release, and an SquareSoft employee said it won't happen because the PS2 IME is ~$60 per license, and they don't want to pay that for each and every non-JP version they sell. It would also trigger more tech support calls, costing SE more money, when people that don't know anything about Japanese accidentally hit the toggle and switch into JP input mode and don't know the keystrokes to get out of it.

The only given answer is to install the JP version and use that when you need to type Japanese.