In this screen shot, the Hijri calendar is being selected for use in an American English (i.e., “en_US.UTF-8”) locale.
Let's compare this feature with Windows XP. Windows XP also lets you choose the Hijri calendar: however, to use it, you must first select an Arabic locale. KDE does not impose such a restriction. The fact that the Hijri calendar is not dependent on an Arabic locale in KDE is a natural outcome of the modular design of almost every facet of Linux.