+1 for this.

Preview for translated front pages shows latest post listing instead of the front page.

Here is a temporary fix for this issue, which adds post name to the front page preview url, when redirect_lang option is enabled.

/**
 * Fix broken front page preview link in Polylang, when redirect_lang option is enabled.
 *
 * @param $preview_link
 * @param $post
 *
 * @return mixed
 */
function valu_fix_front_page_preview_link( $preview_link, $post ) {

	// Check that post type is page
	if ( 'page' !== get_post_type( $post ) ) {
		return $preview_link;
	}

	// Check that redirect_lang option is enabled
	if ( 1 !== PLL()->options['redirect_lang'] ) {
		return $preview_link;
	}

	// Get language of the current page
	$language = PLL()->model->post->get_language( $post->ID );

	// Check that current page is a translation of the default front page
	if ( $post->ID === $language->page_on_front AND pll_default_language() != $language->slug ) {

		// Get front page link with postname
		$preview_link_with_postname = esc_url( trailingslashit( get_permalink( $post->ID ) . $post->post_name ) );

		// Apply new preview link
		$preview_link = str_replace( PLL()->links_model->home_url( $language ), $preview_link_with_postname, $preview_link );

	}

	return $preview_link;

}

add_filter( 'preview_post_link', 'valu_fix_front_page_preview_link', 30, 2 );