Index: src/wp-admin/options-general.php
===================================================================
--- src/wp-admin/options-general.php	(revision 29682)
+++ src/wp-admin/options-general.php	(working copy)
@@ -305,7 +305,10 @@
 
 <?php
 $languages = get_available_languages();
-if ( ! empty( $languages ) ) {
+if ( ! is_multisite() && defined( 'WPLANG' ) && '' !== WPLANG && 'en_US' !== WPLANG && ! in_array( WPLANG, $languages ) ) {
+	$languages[] = WPLANG;
+}
+if ( $languages ) {
 	?>
 	<tr>
 		<th width="33%" scope="row"><label for="WPLANG"><?php _e( 'Site Language' ); ?></label></th>
Index: src/wp-includes/formatting.php
===================================================================
--- src/wp-includes/formatting.php	(revision 29682)
+++ src/wp-includes/formatting.php	(working copy)
@@ -3361,8 +3361,12 @@
 
 		case 'WPLANG':
 			$allowed = get_available_languages();
-			if ( ! in_array( $value, $allowed ) && ! empty( $value ) )
+			if ( ! is_multisite() && defined( 'WPLANG' ) && '' !== WPLANG && 'en_US' !== WPLANG ) {
+				$allowed[] = WPLANG;
+			}
+			if ( ! in_array( $value, $allowed ) && ! empty( $value ) ) {
 				$value = get_option( $option );
+			}
 			break;
 
 		case 'illegal_names':