One cashier reads Arabic and another reads English. Can each of them have the POS in their own language?

The language belongs to the device, not to the person. Each paired device has its own language setting for its screens, so a tablet can be in Arabic and another in English. The receipt's language is chosen separately under Settings → Printing & receipts and applies to the whole restaurant. If two people share one tablet, they share its language too. The POS and the counter tab in the docs explains the language settings.