Have you checked your keyboard settings? If it's like Android, I would go to Settings > Language & input > "select your keyboard" in Keyboard & input methods > Languages, and choose your language.

I am going to flash Firefox OS to my Nexus 5 soon to check.

Please go to settings > Keyboard Settings then download the Arabic keyboard. When typing anything you will see a button named "En" on your keypad keys. Press and hold the button to see other installed Keyboards. Then selecet Arabic from those. Your keyboard keys will turn onto Arabic keys.

Feel free to ask if you face further problems.