We now support RTL... sorta... on two sites.
On Mi Yodeya (Judaism) and Biblical Hermeneutics, we have an integrated virtual keyboard for entering Hebrew. It includes a button to insert a right-to-left mark, which signals the browser to display text, well, from right to left. For sites that are mostly English, but also include sources in a RTL script, that should be sufficient. Currently, we do not support Islam with an Arabic keyboard, but depending on how the experiment goes on the Hebrew keyboard, that's a possibility.
Looking ahead, there is also a Hebrew proposal. It's unlikely that we'd offer the virtual keyboard since we would expect that people using or learning Hebrew would have their own methods for entering RTL texts. Since the site hasn't gotten close to clearing the commitment bar, we haven't considered other aspects of the site that might be comfortable to a Hebrew reader. (See Wikipedia in Hebrew to get an idea of what that might be like.) As Stack Exchange increases its scope internationally, this is certainly something that we'll need to consider more carefully.