We're skipping support for plugins / themes this time around. When core supports plugins / themes in a more substantial way, we'll likely move wp core language * to a top-level wp language command.

By @danielbachhuber in #1382 (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.