Given that a large portion of the site's target demographic probably do not speak English as a first language, would questions/answers in other languages (e.g., Urdu, Hindi, Arabic) be accepted?
Stack Exchange sites are defined by communities, not merely by topics. If a group of people are interested in a topic but are unable to communicate with each other then they do not constitute a real community for a Q&A site. Therefore I think the main part of the questions should be written in English.
Other users can help the OP in stating the question in English correctly. Note that this is not an issue only for this site, many users on other SE sites have the same problem of not being native English speakers.
On the other hand, quoting Quran and other references in Arabic (and in their original language, whatever it is) will be required for stating some questions, so it should be fine to quote in another languages. But even in such cases the OP should try to provide an English translation (preferably from a respected source and not a self-translation/interpretation).
The site doesn't have to be in English, but splitting the site by language would not be a good idea. The site needs to pick a single language, whether English, Arabic, Urdu, or whatever.
We don't want to have questions that have to be duplicated solely because they were asked in a language that the person wanting to ask the duplicate doesn't speak. It's best to pick the single language up-front and stick to it.
This doesn't mean that it can't use other languages, of course. But entire questions and answers shouldn't be in anything but the site's language of choice.
I suggest foreign language questions to be closed, unless they are translated into English in a few hours. And, the users should be patient while closing foreign language questions; they shouldn't vote a foreign language question to close if it is asked just one hour ago.
That might not always be possible not every Muslim knows English so if they can provide the translation so that people can understand it, picking a single language is almost impossible.
In my opinion, people could voluntarily add translations for questions and accepted answer and/or answer with highest rating for languages they know. This way more people can get benefit. Like below: