ThMS is shutting down

You mean the costs that they'd be charging the franchises professional services fees for, that would then be considered 'profit' for them?

Those professional services might not be cost effective enough to warrant releasing a game to a single region, let alone updates and patches. On Nexon Korea's part, I don't think alienating potential investors with an exorbitant localization fee on top of the franchise fee is what they would want which is why I think they would want the "English only" condition.

It's either quality assurance, cost effectiveness, or just code incompatibility as another person mentioned... otherwise I have no idea why they would want the "English only" condition.