We are managing WordPress core through composer (https://packagist.org/packages/johnpbloch/wordpress-core), so it is not a localized wp package, but often install it in another language than the default en_US. Historically we have then been including translations through composer packages from https://wp-languages.github.io/ (which we probably don't need any more with all the improved language functionality in wp-cli).

However, our problem relates to the install process. Since we don't have use a localized WordPress package (eg that wp core download --locale=sv_SE would have given us), when we run wp core install it will be setup using en_US language.

We can also not run wp languages core activate sv_SE before the install since that requires wp core install to be run first.

We can obviously install/activate another language after installation but that then means that in the installation process, which typically creates a post/page/comment/default category, our content will still be in en_US.

So my suggestion would be to allow wp core install to be used with a language/locale parameter so a site can be installed with a specific language without using a localized wp core package.

I think it makes sense since the actual wp_install function in wp-admin/includes/upgrade.php also got this parameter (think it was added in to WP 4.0).

This would then mean that we could run:

wp language core install sv_SE
wp core install --locale=sv_SE

and then have a site which also have content in the correct language.