Steps to reproduce: see URL.
$wgDefaultUserOptions['lqtnotifytalk'] = true; 
on translatewiki.net, a user will receive email notifications for new threads or replies to threads if the thread is watched (that is, in LQT jargon, if it's individually watchlisted or its parent [talk] page is), regardless of their email notification settings, i.e. even if one or both of "E-mail me when a page or file on my watchlist is changed" and "E-mail me on replies to a thread I am watching" are disabled.
The only workaround is to unwatch both the page and the thread.