while you wait, please have a short read
http://forums.scribus.net/index.php/topic,3681.msg17351.html#msg17351in short: you need to define the tabs as if your text was LTR and it will behave correctly in RTL (but all rulers are wrong).
the question is: how should it work for RTL languages?
should we invert the content of the "widget"?
what should happen with the existing tabs, when one switches from LTR to RTL (and vice versa)?
if you or somebody you know can help with that maybe scribus can improve...
if you know somebody who could write a patch, that would be wonderful!
p.s.: what you are marking in your screenshot is just the setting if the tab itself will be a left or a right one.