The poster is in English. Is that their language? Or is it staged for the American press?
American press. Some Iranians know English, though.
I’d say it’s for the entire non-Farsi world, actually. The protesters don’t have a lot of levers to work with, but outside support is a straw they can at least grasp at. The most widely spoken language in the world is English and even many non-speakers have modest understanding of written English... Farsi, not so much.
(Spoken Chinese is a close 2nd to English, but I rather suspect Chinese on the poster won’t do the young lady much good!)