I've never tried Gentoo, but I hope these instructions are generic enough for you to get something useful happening.
You will need to make sure you have Arabic locale files for KDE installed, they should be somewhere on your install disks, the file name will be something like kde-i18n-ar-version
.You will also need some Arabic fonts.
Now, go to the KDE configuration menu and from the Accessibility submenu select Keyboard Layouts. Check the enable keyboard layouts, select "ar" as an option and then you can toggle this from the applet at the bottom right hand corner next to the clock on the KDE toolbar...
Hope it works!