Unable to change backend language During install, it is possible to choose the language to be used on the admin screens. However, in this new Drupal 7.40 instance it was not chosen at install time. I need now to change the language from the default English (this is NOT a multilanguage issue).

Every tutorial I read tells me to click the "Add Language" button on Administration > Configuration > Regional and language > Languages. But there is no such button, there isn't even a "Languages" option under "Regional and language":
The option is displayed only after activating the "Locale" core module on the admin > modules page.