diff --git a/wp-admin/install.php b/wp-admin/install.php
index bf102893ff..34d5d4c7f7 100644
--- a/wp-admin/install.php
+++ b/wp-admin/install.php
@@ -329,7 +329,7 @@ if ( defined( 'DO_NOT_UPGRADE_GLOBAL_TABLES' ) ) {
  */
 $language = '';
 if ( ! empty( $_REQUEST['language'] ) ) {
-	$language = preg_replace( '/[^a-zA-Z0-9_]/', '', $_REQUEST['language'] );
+	$language = sanitize_locale_name( $_REQUEST['language'] );
 } elseif ( isset( $GLOBALS['wp_local_package'] ) ) {
 	$language = $GLOBALS['wp_local_package'];
 }
diff --git a/wp-includes/l10n.php b/wp-includes/l10n.php
index 67bfb1a26e..598a485468 100644
--- a/wp-includes/l10n.php
+++ b/wp-includes/l10n.php
@@ -150,6 +150,15 @@ function determine_locale() {
 		( isset( $_GET['_locale'] ) && 'user' === $_GET['_locale'] && wp_is_json_request() )
 	) {
 		$determined_locale = get_user_locale();
+	} elseif (
+		( ! empty( $_REQUEST['language'] ) || isset( $GLOBALS['wp_local_package'] ) )
+		&& wp_installing()
+	) {
+		if ( ! empty( $_REQUEST['language'] ) ) {
+			$determined_locale = sanitize_locale_name( $_REQUEST['language'] );
+		} else {
+			$determined_locale = $GLOBALS['wp_local_package'];
+		}
 	}
 
 	if ( ! $determined_locale ) {
diff --git a/wp-includes/version.php b/wp-includes/version.php
index 2163dbf807..15b18f035a 100644
--- a/wp-includes/version.php
+++ b/wp-includes/version.php
@@ -16,7 +16,7 @@
  *
  * @global string $wp_version
  */
-$wp_version = '6.5-alpha-57285';
+$wp_version = '6.5-alpha-57286';
 
 /**
  * Holds the WordPress DB revision, increments when changes are made to the WordPress DB schema.