As I understand it, we simply need someone to keep the LanguageAr.php file up to date to localize the relevant keywords. When MediaWiki provides the _functionality_ to localize something, but it is not localized because a volunteer hasn't done it, it isn't MediaWiki that is broken. We just need people to maintain the language files in each language; whether it's LTR or RTL is irrelevant. This process can of course be expedited by paying someone to do it.
As for distinguishing between minor annoyances and things which are "horribly broken", I'd like to see some surveys among the actual users of the various non-English Wikipedias. As things are, I am entirely unconvinced that it is RTL issues which are the cause of the fact that ar.wikipedia.org has "only" 12000 articles. Limited access to or use of the WWW would be a much more important factor.
Erik