Here's why:
* Historical context: In the 12th century, the Levant was a melting pot of languages. Arabic was the dominant language, but other languages like Hebrew, Greek, Armenian, and even some European languages were spoken.
* Game's narrative: The game focuses more on the story and doesn't delve into linguistic details. Altair interacts with characters speaking different languages, but his dialogue is in English (or the player's preferred language).
* Artistic license: Video games often take liberties with historical accuracy for storytelling purposes.
Therefore, while it's plausible Altair spoke Arabic, it's also possible he spoke other languages or a combination of them. The exact language is left open to interpretation.