The translation language cannot be changed

Problem/Motivation

I have imported translated content from a Drupal 7 instance in English (en-gb) and I want to change to English (en)

When selecting the language, the error below is displayed :

The translation language cannot be changed (en-gb). in Drupal\Core\Entity\ContentEntityBase->onChange() (line 851 of core/lib/Drupal/Core/Entity/ContentEntityBase.php).

Steps to reproduce

Proposed resolution

Remaining tasks

User interface changes

API changes

Data model changes