Plone 5.2 does no longer install a Language
index...bug or feature?
Installing plone.app.multilingual
sets up the Language
index.
Gently, I would call it a feature. In monolingual sites the index is just dead ballast. So, there are probably use-cases where someone needs it w/o p.a.multilingual. If its related to an alternative add-on I would suggest to make the add-on to set it up on its own.
Mea culpa...