[Plugin: Multilingual Press] Is there a way to "untranslate" posts?

Here’s my problem. I converted an existing page into a multisite to make the page multilingual. Then I started to translate existing posts by checking the little box on the editing area of the post which created a draft on the subpage, everything fine to this point.
Unfortunately my boss later decided to add another language and it’s not possible to link the posts to this second language. Getting “no preview available” in the editing area of the posts.
So, I would like to know if there’s a way to “untranslate” the existing posts from the mainpage to start over. Just reinstalling the plugin didn’t do the job.

Thanks in advance
TK

http://wordpress.org/extend/plugins/multilingual-press/