Create a site called "Sólo Español", set its default language to es_ES and remove any other language from the list, so that only es_ES exists.
Now, create a web content in that site.
Observed: message error is displayed, content is not created
Expected: content should be created with no errors
Tested in master at 6d30aff6c2aebac750c630c0675d00eed025ca97