Summary:
There should be a slot for Levantine Arabic in the timed-text options for Commons and Wikipedia.
Though they use the same script, Levantine Arabic and [Standard] Arabic are not mutually intelligible.

Benefits:
Caption support for 50 million speakers.

Note:
Manually changing the language code in the URL when adding captions from any selected langauage to that of Levantine Arabic (apc) properly displays it as Levantine Arabic, but the captions don't show in the video player: example.