You need a backend css redesign to be RTL - this should be done in the core files.
Check this link for updates
http://www.we-cms.info/blog/The guy's name is Erez Wolf I suppose he will eventually release the new backend translation for 2.7 Look at the cms directory there
Personally I use english version of WB for myself and only use hebrew for clients.
Nontheless, you can download the tinyMCE module it has great RTL support and you can translate (or get translation) of Hebrew language file (Erez made a translation back for 2.6.4).
I can send you the hebrew translation of 2.6.4 which is about the same in 2.7 but your backend will remain LTR. If you want the file drop me a message.