Understanding Wordpress Default Theme - twentyfifteen.1.4 >> Languages Folder

Understanding Wordpress Default Theme - Twentyfifteen.1.4 is the standardized theme designed by the wordpress people. in the language folder there is a template. just like we have .php extension or .js or .css No extension is showed there. it Just says Type: Translation Type.

Please see this snapshot - http://screencast.com/t/s08NswYAzmJ4 can some one please help me to understand what extension type is this and how can one create this?

Alternatively you can also download twentyfifteen.1.4 theme from here.