PlaceholderAdmin's template should include the same language tabs as the PageAdmin has, otherwise languages cannot be added via the Admin.
PlaceholderAdmin is currently "dumb" when it comes to languages. I'm not entirely sure how to solve this, but I know there's interest in this (cc @evildmp )
Should it be a Placeholder that has languages? Or should it be the thing that has a Placeholder that has languages?
Not sure, we could have one placeholder per language, or do it on the plugin level or on the placeholder field level. Not sure to be honest.
I'm not deeply familiar with the whole language changing business, but an approach would be to aggregate all PlaceholderFields in to a fieldset on the change page. Then we could put some language tabs around the top of that fieldset and maybe save the selected language via ajax. the language tab could simply change the form with some js (à la bootstrap-tab.js from twitter bootstrap). That way we could partly decouple the django-cms stuff from the other fields on that model. I'm not sure if we even have access to all language data when we load a model with a PlaceholderField...
fixed in develop