It’s a two-stage delivery: the first stage is noticing Donald Trump’s name next to some Arabic. That raises a question in the (Anglophone) viewer’s mind anchored to a specific item. That arouses their attention for the explanation when they seek it out or come across it, which delivers the joke: that it’s designed just to arouse Trump’s paranoia.
Just putting up some Arabic won’t arouse the same curiosity, may even work against it: it would just be a random bit of foreign gibberish, no more significant than an ad for dry-cleaning. The viewer wouldn’t seek out an explanation, and might not even link an explanation overheard to the ad.
