Most people who watch Korean dramas are from Korea, mainly the southern part. But Korean dramas are becoming extremely popular all over Asia. China, Japan, Vietnam, etc. They're translating them into their languages and putting them on television in their countries. A lot of korean drama stars are well known all over Asia, as well.
As for America, I watch them, and some of my friends do, too, but they're not really well-known in the United States. I mean, some people watch them online, but they're not so popular that they're put onto TV daily.
So yeah, I think it'd be safe to say that most people who do watch korean dramas are either Korean or speak Korean.
~Hope this helps! :)