For Saudi Arabia, yes — Arabic-first creative and copy should be in place from launch, since the audience is majority Arabic-speaking and expects it. For the UAE, English-only can work at launch for expat-heavy B2B or premium consumer categories, but it caps your addressable audience.
The safer default across the GCC is bilingual from day one, even if English carries more weight early in the UAE. Arabic is rarely optional for long; it is a question of sequencing, not whether.