It's important to note that:
* Arabic is not a single language, but a family of languages. There are many dialects spoken across the Arab world, some of which are mutually intelligible, while others are quite distinct.
* Modern Standard Arabic (MSA) is the standardized form of the language used in formal settings like government, education, and the media. It's based on classical Arabic, the language of the Quran.
* While most Arabic speakers can understand MSA, their everyday communication is usually in their regional dialect.
So, while the answer is "Arabic," it's a complex answer with nuances depending on how you define "language."