Reworking content_translation_enabled()

Problem/Motivation

Followed up #2328161-24: "Enable Translation" checkbox default non-predictable for new bundles we need some changes on content_translation_enabled()

The current documentation of content_translation_enabled() is not clear ("Determines whether the given entity type is translatable"). It should be something like "Determines if a specific bundle is translatable. If no bundle is given the function returns if at least one of the entity types bundles is translatable." But we are not sure if this is what the author intended.

Proposed resolution

Remaining tasks

User interface changes

API changes