fn set_html_lang(locale: &str) {
        let document = web_sys::window().unwrap().document().unwrap();
        // If the `<html>` tag does not exist, and Perseus is running, this would
        // be...interesting...
        let html = document.document_element().unwrap();

        // This is a non-critical operation, so errors are not propagated here
        // TODO In future, this should integrate with the `PlatformError` system
        let _ = html.set_attribute("lang", locale);
    }
Sets the `lang` attribute on the root `<html>` tag to stay up-to-date
with the internal state of this system.
packages/perseus/src/i18n/client_translations_manager.rs