It was added in 3f1c7b70.

Single language sites should always be translated in LANGUAGE_CODE,
regardless of the browser's Accept-Language. Having LocaleMiddleware
enabled can result in having some parts, like the admin, translated
in an unexpected language, typically if someone browses a non-English
website on a system set up in English. Since most websites won't be
translated in multiple languages — especially at the time they're
created — it's better not to enable LocaleMiddleware by default.

Thanks Ramiro for the feedback.