This is a quick survey of the open source world to find platforms that might be suited for bringing up blog services or news hub sites in Middle Eastern languages. The best fit depends on whether the goal is a large, hosted service a la Blogger and Typepad, and a more stand-alone system that would support one to a few blogs, or be the core of a newshub operation (a la OhMyNews).
In the large, hosted category, the only real contender is LiveJournal. (Hat tip: Michael Edwards) This is open source code for the system driving the service of the same name. How many of the many scalability features descibed in this PDF slideshow are included is unknown, I haven't dug into the code base as yet. LiveJournal has an active developer base. It is multibyte character capable, and there are a number of teams engaged in translations, however none have yet completed and brought up a non-English version. There is a conference area for translation activities. The projects underway do not include Arabic, Farsi, or Urdu. The level of effort for translation, and the division between the server and (optional) authoring client is unknown.
For standalone, the leader is Mambo, which is really positioned as a full-up Content Management System, with blogs being one use. There is commercial version of Mambo. (And for Mac heads like myself, it's said to run well on OS X.) In this cautionary post on its capabilties, 'multilingual' appears to multiple languages at once, as the same poster cites his work on a German version for a commercial project. There also seems to be an Arabic version, Al Mambo, put up by a small Michigan firm that appears to have ties to Tunisia. (If any Arabic speakers are reading, a comment on this project would be appreciated.)
Finally deserving mention is WordPress. This appears to be a relatively new project, targeted at a standalone site. It lacks the feature sets of Mambo and the numerous scalability enhancements of LiveJournal. There are no translation efforts underway as yet.