sort (GNU coreutils) 8.26
local of the system: en_US.UTF-8

contents of /etc/locale.conf:
LANG=en_US.UTF-8
LC_TIME=fr_FR.UTF-8
LC_NUMERIC=fr_FR.UTF-8
========================================
$ echo $'䜌\n龹' | sort -u

Expected output:
龹
䜌
Actual output:
䜌