I'm sure Udemy will market your course in Greece if the algorithm thinks your course will sell there. But obviously, bringing in some traffic on your own will be helpful. Also considering that the market for Greek courses is way way smaller than the market for English courses.
Perhaps you can think about re-creating your course in English? It could be the same material but in English. That would allow you to offer your courses to a wider market. I'm not sure what the official policy is about re-creating a course in a different language, but from what I hear, it's done fairly often.