Function compile_site_with_locales 

pub fn compile_site_with_locales(
    build_dir: &Path,
    content_dir: &Path,
    site_dir: &Path,
    template_dir: &Path,
    base_url: Option<&str>,
    locales: &[String],
) -> Result<(), SsgError>

As compile_site_with_base_url, plus the configured locales so the content stager can derive locale_path / locale_url per page.

Separate from the public entry point so that signature stays stable for embedders; the pipeline itself always calls this one.