Arabic is a Semitic language; other closely related Semitic languages include Amharic (Ethiopia) and Hebrew. More distantly related are languages like Berber (North Africa) as well as historic languages including Phoenician and ancient Egyptian.
English is the most commonly taught second language in the country and most college educated people speak at least some English. Some industries are heavily reliant on international trade, or are taught in English so English is again stressed and fluency within these industries is common.
Religion
As the center of the Muslim world, it is not a surprise that Muslim is the official religion of Saudi Arabia and all citizens must be Muslim. Although foreigners may live in the country temporarily, if a non-Muslim dies in the country, their body must be removed for burial. By law, every citizen of Saudi Arabia is Muslim.
Islam (the name of the religion, whose followers are called Muslims) is a monotheistic religion, whose holy book is called the Qur'an. The Qur'an is believed to be the word of God spoken through the prophet Muhammad from 609-632 CE (Common Era is preferred over AD (Anno Domini or "year of the Lord") since the Islamic world doesn't believe Jesus was the messiah). Islam believes Muhammad was the last prophet sent to earth by God, the last in a long line of prophets, which includes Moses, Abraham, and Jesus among others.