Don't forget the support issues. Imagine if a player unwittingly presses alt+shift
"What the hell? What are these funny squiggly characters? How do I get out of this?"
He tries contacting a GM, but he can't type because he's still in the JP IME.
He tries logging out and logging back in but it's still the same issue.
By now he's thoroughly frustrated at possibly having to reinstall the game.
Don't think this is possible? Having worked at a helpdesk before, this is an entirely plausible scenario.
Enabling JP support = catering to a vanishingly small portion of the NA playerbase, along with massive support issues. Not happening.