Twitter is popular outside the United States, but so far Twitter has been slack in offering true international support, at least outside of Japan.
Character recognition isn't an issue for Twitter, but interface language and support for languages that run right to left are, particularly Arabic and Persian.
Artwitter.com uses the Twitter API to deliver a Twitter interface in Arabic, that supports right to left entry of Tweets. Artwitter.com includes the main friend timeline, replies and posting support, the same functions available on Twitter itself, through a simple right-to-left Arabic interface, without storing any user data on its server.
StatupArabia notes that the site is still in early beta, and there are still some features missing, such as the inability to view another users timeline or search for tweets, but will be added later if the service becomes popular.