If a Drush 9 commands allows arguments, the argument 'en' will be replaced with its full command name pm:enable. In this way it isn't possible to create an argument that accepts the language code 'en'.
Drush 8 doesn't have this issue.
Some test cases:
$ drush @site_alias en 'en'
[error] Unable to install modules pm:enable due to missing modules pm:enable.
$ drush @site_alias pm:enable 'en'
[error] Unable to install modules pm:enable due to missing modules pm:enable.