Index: wp-admin/includes/upgrade.php
===================================================================
--- wp-admin/includes/upgrade.php	(revision 35516)
+++ wp-admin/includes/upgrade.php	(working copy)
@@ -155,11 +155,15 @@
 	if ( is_multisite() ) {
 		$first_post = get_site_option( 'first_post' );
 
-		if ( empty($first_post) )
-			$first_post = __( 'Welcome to <a href="SITE_URL">SITE_NAME</a>. This is your first post. Edit or delete it, then start writing!' );
+		if ( empty($first_post) ) {
+			$first_post = sprintf(
+				/* translators: 1: site URL 2: site name */
+				__( 'Welcome to <a href="%1$s">%2$s</a>. This is your first post. Edit or delete it, then start writing!' ),
+				esc_url( network_home_url() ),
+				get_current_site()->site_name
+			);
+		}
 
-		$first_post = str_replace( "SITE_URL", esc_url( network_home_url() ), $first_post );
-		$first_post = str_replace( "SITE_NAME", get_current_site()->site_name, $first_post );
 	} else {
 		$first_post = __( 'Welcome to WordPress. This is your first post. Edit or delete it, then start writing!' );
 	}