Index: src/wp-admin/install.php
===================================================================
--- src/wp-admin/install.php	(revision 29696)
+++ src/wp-admin/install.php	(working copy)
@@ -184,6 +184,8 @@
 $langugage = '';
 if ( ! empty( $_REQUEST['language'] ) ) {
 	$langugage = preg_replace( '/[^a-zA-Z_]/', '', $_REQUEST['language'] );
+} elseif ( isset( $GLOBALS['wp_local_package'] ) ) {
+	$language = $GLOBALS['wp_local_package'];
 }
 
 switch($step) {
Index: src/wp-admin/setup-config.php
===================================================================
--- src/wp-admin/setup-config.php	(revision 29696)
+++ src/wp-admin/setup-config.php	(working copy)
@@ -90,7 +90,10 @@
 $language = '';
 if ( ! empty( $_REQUEST['language'] ) ) {
 	$language = preg_replace( '/[^a-zA-Z_]/', '', $_REQUEST['language'] );
+} elseif ( isset( $GLOBALS['wp_local_package'] ) ) {
+	$language = $GLOBALS['wp_local_package'];
 }
+
 switch($step) {
 	case -1:
 		if ( wp_can_install_language_pack() && empty( $language ) && ( $languages = wp_get_available_translations() ) ) {