add-form entity links require special handling - set the current content language

Problem/Motivation

If we generate a link for the add-form link template, this will currently throw an exception in strict mode.

Proposed resolution

We need a special handling for the add-form link template, where we have to set the current content language on the URL being generated. Additionally we have to ensure that the current content language will be preselected in the language select widget for new entities.

Remaining tasks

User interface changes

API changes

Data model changes