Khatun (the lady Prairie meets in her near-death-experience and who takes her sight away) speaks the original Arabic language most of the time.
Why did the writers choose Arabic for this character? Does it symbolize anything?
We don't know
Like most things in The OA, there is all kind of symbolism which is mostly left up to the viewer to parse out and/or decide for themselves.
There hasn't been any comments from the show's creators on specifically why they made Khatun Arabic, but the word "khatun" means "queen" or "empress".
My guess is that the creators were attempting to give her a new age, mystical aura while being somewhat cryptic about her ultimate identity. She also has elements from other cultures for example:
The following article does a good job of pointing some of these characteristics out and giving some theories on Khatun's ultimate role for the show.