This approach seems to work well now, drastically reducing the deployment time when languages should be installed and also preserving site-specific language files (in site git repository). Also there is no short period of time during deploy anymore where the language falls back to default language (English).
roles/deploy/defaults/main.yml (at line 14):
# There are certain folders you'll want to copy from release to release to speed up deploys.
# Examples: Composer's `vendor` folder, npm's `node_modules`.
# These should not be part of project_shared_children since dependencies need to be atomic and tied to a deploy.
project_copy_folders:
- vendor
- web/app/languages
Add web/app/languages to project_copy_folders list.