I found a little trick through which you would be able to change the default Arabic font in OS X. I have only tested it Lion so, I don't know if it would work in 10.6 and earlier versions. This trick should probably work for other languages too; with English you can just use TinkerTool.
The default OS X font for Arabic is Geeza Pro which is more like Times New Roman is for English. I wanted to have a better looking font, the one I had in mind was Tahoma.
I discovered that when you disable Geeza Pro, OS X automatically sets the next available font that supports Arabic as the default font. And since the fonts are organised alphabetically, the next available font was Arial.
Like I said I wanted to use Tahoma as my default font, so I had to modify its name to something like AAATahoma, so that its the first available font when Geeza Pro was disabled.
Modifying the name, the way you would change the name of any file in OS X will not do the trick, since there are other names embedded in the font that also need to be modified. In order to achieve that you will need to use a font editor to modify the PostScript name, full name, and family name of the font, so that OS X recognises it as a new font, otherwise you will have a duplicate conflict with the original Tahoma font.
A friend of mine had Fontlab studio, and it was fairly easy to modify the name. Fontlab is a paid app, but I think you can use the free Fontforge app to do the same thing.
Once the font name was modified to AAATahoma and AAATahoma Bold, I exported it and then added it to Font Book and there was no conflict with the original Tahoma font.
Now all you have to do is disable Geeza Pro and then restart your machine, or just restart the apps that you want to use the new default font.
I have been using this for a couple of days now and I haven't had any issues with all of my apps except the official Twitter app, which has become a bit unresponsive, and at times the fontd
process jumped to 98% CPU usage for a few seconds when there were a couple of Arabic tweets to load. Other than that, all of my apps, like Chrome, Safari, TextEdit, WriteRoom, Finder, Mail etc are working absolutely fine.
Note: some apps like the iWork suite will not support some Arabic fonts other than the ones provided by Apple.
is a screen shot of both Tahoma and the New AAATahoma fonts in Font Book.
I haven't tested this one. Let me know if it works on earlier versions of OS X and I will edit things accordingly.]