You didn't setup any blog relationships! You have to setup them first to use Multilingual Press. For this, go to Network » Sites » and choose a blog to edit. Then go to the tab "Multilingiual Press" and set up the relationships.
This is a maessage I get when wanting to update to 3.5.1
I do not have any blogs to edit, I don't need a blog on my site.
How do I move on to use Multilingual Press?