phpBB is translated in more than 50 languages, it should be the same for the CLI.
Needs more work.
https://github.com/phpbb/phpbb/pull/2547 was actually not merged. https://github.com/phpbb/phpbb/pull/2752 was merged.
The existing CLI language strings will have to be moved into cli.php before 3.1.0-RC1. Upgrading to Blocker.
It might be possible to reuse https://github.com/phpbb/phpbb/pull/2547.