Greetings,
I checked the git for any closed issues and found Issue #1672 and Issue #1770, as @ben said we could still discuss this, Iโm opening it here
I believe #1770 makes more sense and does not necessarily mean we should provide a default .po/.mo file, as @ben said it is not really the point of a starter theme, however is the theme correctly uses gettext calls it should also load the textdomain.
This is quite simple to do, still it belongs on a starter theme as being almost required on all projects.
I suggest adding it to app/setup.php:110
'dir.language' => get_stylesheet_directory() . '/lang/',
And after that:
/**
* Make theme available for translation
*/
load_theme_textdomain('sage', $paths['dir.language']);
And creating a lang folder, not really needed to add a .po file in my opinion.