How can I specify language for Parent Item options on node create/update page?
I have a similar problem, I have decided that I don't want to ajax reload the form when the language is changed because things like entity relationships (which is my current problem) don't work even then.

My advice is to force the language of the editing / adding node to be the same as the interface. For creating/ adding this is easy you set it for the content type when you edit the multilingual settings. The translation interface redirects correclty and the content creation too (it expects you to be in the language you want to create). 

So with that option you would be on the safe side, EXCEPT when you are in pages like "find content" and use the edit link or write the url by hand. 
What I am trying to do now is to force a redirect when you try to edit the link by a url like /node/223/edit. This is something that should be possible with hook_menu_alter.I would force a redirect to [language of the content]/node/223/edit.

I would have thought that there was a better possibility but I have found none. I will be monitoring this question to see if someone has a better idea or there is a much better solution out there.