This probably explains things better than I could. Notice the focus on the content of the gospel vs. who is the intended reader ...
Audience of Mark's Gospel
Audience of Mark's Gospel
For Whom Was the Gospel According to Mark Written?
...
First, Mark was written in Greek rather than Aramaic. Greek was the lingua franca of the Mediterranean world of that time, while Aramaic was the language common to the Jews. Had Mark been interested in addressing Jews specifically, he would have used Aramaic. Furthermore, Mark interprets Aramaic phrases for the readers (5:41, 7:34, 14:36, 15:34), something that would have been unnecessary for a Jewish audience in Palestine.
Second, Mark explains Jewish customs (7:3-4). Jews in Palestine, the heart of ancient Judaism, certainly didn’t need Jewish customs explained to them, so at the very least Mark must have expected a sizable non-Jewish audience reading his work. On the other hand, Jewish communities well outside Palestine may not have been familiar enough with all the customs in order to get by without at least some explanations.