diff --git a/src/wp-includes/l10n.php b/src/wp-includes/l10n.php
index 0fbc57f3c4..8918b58ffb 100644
--- a/src/wp-includes/l10n.php
+++ b/src/wp-includes/l10n.php
@@ -106,7 +106,7 @@ function get_user_locale( $user_id = 0 ) {
 	}
 
 	$locale = $user->locale;
-	return $locale ? $locale : get_locale();
+	return ( $locale && in_array( $locale, get_available_languages() ) ) ? $locale : get_locale();
 }
 
 /**