[Feature Request] List missing translations endpoint

[sneridagh]

Given a multilingual site, an endpoint that got through all existing multilingual content and checks if there is any Translation group with missing translations on it.

One could also use to have an endpoint that creates a map of the existing state of the translations in the site in order to assess them.

In big sites this information could be quite big, we should think also in a way to batch and filter the values returned.

As a frontend, we could think in a control panel very much the same as the aliases one.