[plone.app.multilingual] Retrieving translations for an content object in Plone 5.1

Plone 5.1.4/plone.app.multilingual: I created a new site with two languages, p.a.m enabled, I added a content in de and translated it to en.

Now I am trying to receive the translations through the API but this fails bady...anything missing here?

>>> doc = app.restrictedTraverse('i18n/de/seite-de')
>>> from plone.app.multilingual.interfaces import ITranslationManager
>>> ITranslationManager(doc)
Traceback (most recent call last):
  File "<console>", line 1, in <module>
TypeError: ('Could not adapt', <Document at /i18n/de/seite-de>, <InterfaceClass plone.app.multilingual.interfaces.ITranslationManager>)

I usually do this check before using the ITranslationManager adapter:

if ITranslatable.providedBy(context):
  tm_context = ITranslationManager(context)

The ITranslatable interface should be added automatically when you add a multilingual behavior to this content type. I assume this is done for the standard content types, when installing p.a.m. Maybe you can check, if there is any of those behaviors?

huh? I used plone.app.multilingual for translating a standard Page...so I would expect that everything else happens upon installation..sorry, can't this right now...