Adapt the redirect target URL fallback (which is the home page by default).

adapt-redirect-url-fallback.php

<?php # -*- coding: utf-8 -*-

add_filter( 'mlp_redirect_url', function ( $url, array $match ) {

	// If this is the home page, any URL is fine.
	if ( is_home() || is_front_page() ) {
		return $url;
	}

	// Prepare the HTTP language code.
	$lang = str_replace( '-', '_', $match['language'] );

	// Build the default home page URL for comparison.
	$home_url = get_home_url( $match['site_id'] );
	$home_url = trailingslashit( $home_url );
	$home_url = add_query_arg( 'noredirect', $lang, $home_url );

	if ( $home_url !== $url ) {
		// The remote URL is not for the home page, so this is good.
		return $url;
	}

	/*
	 * Build your desired target URL and return it.
	 *
	 * If you still want to redirect to a site in your network,
	 * but only want to manipulate the target URL,
	 * you might want to add the noredirect query arg to the URL.
	 * This is done with the following line,
	 * assuming you already set up the URL in $url:
	 */

	return add_query_arg( 'noredirect', $lang, $url );
}, 10, 2  );