from what i can tell this thing happens only after some language update right after a clean flash. it looks like there is something in the Denim rom ( and i tried a couple of theme!!!) that readd the arabic keyboard everytime i restart the phone
I don't know why it happens but it's not only denim related. There are posts where people on cyan and cyan + preview for developer also got these "out of the blue" keyboards.
I know from one of them that the German keyboard did not reappear after a hard rest . He had an UK country variant. And did the hard reset about 2 months ago.