you're /etc/locale.conf seem to be wrong, your LANG should be
LANG=en_US.UTF-8
LC_COLLATE=C
for example this is my output of locale
obarun@ obarun ~ % locale
LANG=fr_FR.UTF-8
LC_CTYPE="fr_FR.UTF-8"
LC_NUMERIC="fr_FR.UTF-8"
LC_TIME="fr_FR.UTF-8"
LC_COLLATE=C
LC_MONETARY="fr_FR.UTF-8"
LC_MESSAGES="fr_FR.UTF-8"
LC_PAPER="fr_FR.UTF-8"
LC_NAME="fr_FR.UTF-8"
LC_ADDRESS="fr_FR.UTF-8"
LC_TELEPHONE="fr_FR.UTF-8"
LC_MEASUREMENT="fr_FR.UTF-8"
LC_IDENTIFICATION="fr_FR.UTF-8"
LC_ALL=
and my locale.conf
obarun@ obarun ~ % cat /etc/locale.conf
LANG=en_US.UTF-8
LC_COLLATE=C
your output of locale -a should be look like this
obarun@ obarun ~ % locale -a
C
POSIX
fr_FR.utf8
so change your locale.conf and regenerate locale by
$ sudo locale-gen
hoping this help
note : i don't use tmux but tried on my system, all seem good