Apparently, there's a message for that: "lqt-reply-summary" [1], but it isn't used there.
As you can see with &uselang=qqx, it doesn't display any message key.
[1] https://translatewiki.net/wiki/MediaWiki:Lqt-reply-summary
Version: master
Severity: trivial
URL: https://www.mediawiki.org/w/index.php?title=Thread:Project:Support_desk/Upgrading_error_ERR_CONTENT_DECODING_FAILED&lqt_method=thread_history&uselang=qqx