This is not possible. Right now, it's on the list of a GSoC student of things to address[1].
[1] https://www.mediawiki.org/wiki/User:Kunalgrover05/GSoc_Proposal
-- Siebrand Mazeland Kitano ICT
M: +31 6 50 69 1239 Skype: siebrand
Op 21 mei 2014 om 10:50 heeft KC Lau kc.kclau@gmail.com het volgende geschreven:
My question is: is there a way to specify the source language for a translatable page when it differs from the site language?