Obviously they had to read some Arabic when studying the Quran, but I doubt that would be sufficient.
On a related note, Charlie Winter published a brilliant essay on the ISIS media strategy on the BBC a couple of months ago, Fishing and ultraviolence[1]. Highly recommended.
[1] http://www.bbc.co.uk/news/resources/idt-88492697-b674-4c69-8...
(edit: removed superfluous and)