"This language is not accepted" error


while adding the title for my website translation I encountered a generic ‘The form was not saved because 1 error(s) were encountered. Please correct these errors and try again.’ error after hitting ‘save’. Upon inspection of the underlying XHR call I see this revealing response:

{"title":{"[\"en_US\",\"gd_GB\"":["This language is not accepted."]}}

It appears as if gd_GB (introduced recently) is creating some issues (I’ve also the ‘default translations’ plugin not showing the sidebar, which might also be linked to this issue).

Screenshot_2020-12-17 Settings

This doesn’t happen on a clean installation, so there probably was something wrong on my database.