Defer "language" preference update

* This avoids DBPerformance log notices on GET requests

Bug: T92357
Change-Id: I9e4bc3fab4cfe36ca820abe8eb46594adac66dc4

UniversalLanguageSelector.hooks.php

@@ -206,10 +206,13 @@ class UniversalLanguageSelectorHooks {
 		if ( !$user->isAnon() ) {
 			// Language change
 			if ( Language::isSupportedLanguage( $languageToSave ) ) {
-				$user->setOption( 'language', $languageToSave );
-				$user->saveSettings();
 				// Apply immediately
+				$user->setOption( 'language', $languageToSave );
 				$code = $languageToSave;
+				// Promise to sync the DB on post-send
+				DeferredUpdates::addCallableUpdate( function() use ( $user ) {
+					$user->saveSettings();
+				} );
 			}
 
 			// Otherwise just use what is stored in preferences