In reply to Brin Wilson.

It is what the WordPress project calls the language specific WordPress sites that can be created.

More info here:

https://make.wordpress.org/polyglots/handbook/translating/requesting-a-new-locale/

For example:

http://es.wordpress.org

http://fr.wordpress.org

It's not just a localized version of WordPress, but the WordPress.org site itself specific to that language.

Comes from https://en.wikipedia.org/wiki/Rosetta_Stone

You've probably heard of the language learning software Rosetta Stone also, which is well known.