500 error when i pick Arabic language from item management

rashedomar

hi all i'm trying to use arabic language for some item
from this link : tao/Main/index?structure=items&ext=taoItems&section=authoring
when i pick the arabic language then press on save i got this error

500: Cannot assign the value to attribute: xml:lang -> (string) ar-arb

how to solve that and how to make the item layout RTL