Originally wp-cli/wp-cli#3782

From @grappler wp-cli/wp-cli#3782 (comment):

We use WP-CLI to install and update translations which works great. The command also installs the theme and plugin translations.

I wanted to see which translations were installed for which plugins. This would be a great extension of the feature.

From @danielbachhuber wp-cli/wp-cli#3782 (comment):

After wp core language is split out to wp language core in #3728, we should add wp language (plugin|theme) for plugin and theme language management.

From @schlessera wp-cli/wp-cli#3782 (comment):

Apparently, when you run wp language core, WP takes care of updating plugins and themes language files automatically now.

We'd probably need to set this filter: https://github.com/WordPress/WordPress/blob/master/wp-admin/includes/class-language-pack-upgrader.php#L74-L82

I just think that WP-CLI should be very explicit about what it does, and what it doesn't. It is okay for WP to hook into an admin backend button click and do some magic. But I don't think that WP-CLI should do that sort of magic as well.