Contrary to what [[mw:Manual:$wgDefaultUserOptions]] says, lqt-watch-threads is unchecked on a fresh account on translatewiki.net. There are no local configs for this.
This is obviously wrong, goes against what intended (per docs), is greatly counter-intuitive and breaks translators signup workflow because translators expect an email notification of some sort after being approved (see URL).
I'm not sure the variable even exists any longer in the code (gitweb's grep didn't find it), maybe it got lost in some refactoring.
However, now that bug 39513 is fixed this should not have nasty consequences.