On the i18n sandbox, I don't see the menu for selecting a language and hence can't select one; except by typing http://i18n.mbsandbox.org/set-language/da or similar directly, after which the selector will become visible. It disappears again when I choose "Reset language".

The reason for this is that, in the reset state, the top-level <li> doesn't contain any text:

<li class="language-selector">
    <ul>
        <li><a href="http://i18n.mbsandbox.org/set-language/af">Afrikaans</a></li>
        ...
    </ul>
</li>

I suspect it is related to my browser configuration, because I don't have any preferred languages set. It works on beta, apparently because English is a hardwired default there.