Gerrit 121615 migrated MwEmbedSupport to use JSON i18n. This was reverted in gerrit 121889 because it caused "PHP Notice: Undefined index: en in /data/project/apache/common-local/php-master/extensions/MwEmbedSupport/MwEmbedResourceManager.php on line 49".
According to MaxSem, this extension implements a home-brewed ResourceLoader. Because I have no knowledge of this, I'm skipping migration to JSON i18n and have reported this bug report. If someone has knowledge of this extension, please make it work with JSON files :).