Stack Exchange started out in English only. But now they are working on support for other languages. It is taking a long time.
At the moment, there is only a single site with an interface that isn't in English: Portuguese Stack OverflowPortuguese Stack Overflow. The interface is in Portuguese only. Stack Exchange is currently working on other single-language sites (I believe that Japanese Stack Overflow is next).
You can certainly use Arabic in messages. This happens often on Islam Stack Exchange.
Non-Latin characters are supported in tags (though there are still a few bugs; I don't know if it would work for Arabic right now).
The missing step that we're impatiently waiting for is to allow users to choose their interface language. All the sites about languages other than English currently allow questions in either that language or English, but having an interface in English only is limiting.