I have been using iBook author, for a time and the simple Mac TextEdit along with openOffice TextDocument. And I recently downloaded Evernote in expectation of more (but that's something else and not writer specific). I also have been looking in an appstore for the same.
I want some suggestion on good text editor that I can use for writing in multiple languages (specially English, Hindi, Arabic, and Urdu). Other language support would be great.
I've been writing on the Mac for a number of years now. Over that time, I've settled into using Scrivener for project organization, version control, and major publishing; and Sublime Text 2 + Markdown Editing for Sublime Text 2 for most of the actual writing. I use Marked for the publishing right now (I mostly blog at Wordpress, and this workflow lets me quickly build reasonably complex blog posts).
Unless you're technically-inclined, I'd recommend just using Scrivener. See this introductory video for a good overview. At its core, Scrivener lets you break up big projects into small chunks, organize them in whatever way suits you, and then get writing.
It's also got powerful version control (I recommend using the Snapshots feature liberally), an attractive full-screen writing mode, and excellent publishing support (output to .doc, .pdf, ebook formats, .txt, MultiMarkdown, and god knows what else). My favorite feature is that Scrivener allows you to bring your research materials directly into the project file, for easy access later. Very useful for term papers.
It uses the standard Mac text-handling engine. If OS X supports the languages you mention, I think Scrivener should too. If in doubt, try the free demo.
The learning curve can be somewhat steep. It's worth it, however, if you're managing a large writing project with a complex structure or one with lots of background information.
If you don't intend to do too much writing, or only write simple projects, you might be better off sticking with Word, Pages, or even just TextEdit at the start. The tool is just that, a tool. If it's getting in the way of your writing, then go back to a simpler tool.
You might try Scrivener. I can't find anything in the documentation about those languages specifically, but it doesn't rule them out either. Free to try for a month, so you've got nothing to lose.
Give a try to IA Writer too. So simple, so beautiful. And with iCloud support you can use iDevices to continue your work where you left off.
Emacs is on the Mac. Just type "emacs" at the terminal.
Emacs supports Unicode and bidirectional text (Arabic), etc.
http://www.gnu.org/software/emacs/manual/html_node/emacs/International.html#International
It also comes with a project planner called org-mode (which also includes its own complete publishing system), artist-mode for ASCII art, dired file manager and a shell so you never have to leave Emacs ever, a browser, the French Revolutionary metric calendar, and a built-in psychotherapist.
You could try out Mellel: http://www.mellel.com/
Apparently it's good at RTL languages which you mentioned you will be using (Arabic, Urdu) and it's cheaper than MS Word etc.
That having been said, I use Scrivener, as various others have mentioned; but I haven't had much experience with multi-lingual documents.
