Can't change content language dropdown setting

I have an english drupal installation on which I created content in spanish. After, it was decided we would start to providing content in english.

The problem is because content was created prior to installing Localizer, all content shows it's language to be english.
I've tried to change each content's language setting from english to spanish, but after saving the changes it goes back to english.

Menu entries appear as created in spanish only after they are modified for the first time while having the interface language in spanish. I've tried this with the content, but everytime I edit a node, even though I start from the spanish interface, it swithces to the content's language (english). I try to change the interface language back to spanish and edit the node to change it's language but it doesn't work.