Steps to reproduce:

1. Create a clean eZ Studio installation;
2. In the backoffice, go to Admin Panel / Languages, and add the language "en/en" (i.e. enter "en" in both "Name" and "Language code" fields and publish). You should now have 2 languages: "eng-GB" and "en";
3. Edit /app/config/ezplatform.yml and add the new "en" language:

ezpublish:
    ....
    system:
        ...
        site:
            languages:
                - eng-GB
                - en

Note: clear symfony cache so that the changes take effect.

4. Create a new landing page e.g. <Test> under the <Home> node and change the language from "English (United Kingdom)" to "en" by clicking on the "change" link on the top-right corner;
5. Publish;

Expected behavior: content is saved in the selected language, and "Content has been published" message is displayed.
Actual behavior: when you click on the "Publish" button nothing happens, and so it's not possible to publish the content. No errors are generated.