Design in Arabic first. English adapts more easily.
Most bilingual sites are designed in English and squeezed into Arabic. It shows, and customers notice.
Arabic words are longer, sit differently on the line and need more height. A layout designed for English text breaks in small ways: buttons wrap, headings lose their rhythm and icons point the wrong way.
When we start in Arabic and adapt to English, the English version almost always fits. Our last six sites took a third less fixing at the end because of this one habit.