Support for the New Era of Japan
While this is a fairly specific addition, it’s extremely important for the users it will impact. iOS 12.3 will introduce support for the next era of the Japanese calendar. Specifically, that’s the Reiwa period, which will officially begin on May 1, 2019 — or when the current emperor’s elder son is expected to ascend to the throne.
The support for the new Era of Japan fixes a fairly major bug that impacted users who had their devices set to the Japanese language. Basically, when a Japanese language user shifted their devices to May 1, 2019, it caused it to essentially freak out. That was apparently due to date format limitations for the Japanese calendar.