Updated my arch linux server and now I get tmux: need UTF-8 locale (LC_CTYPE) but have ANSI_X3.4-1968

The same exact thing happened to me. Building on what Thomas said above, I was able to fix it by uncommenting en_US.UTF-8 UTF-8 in my /etc/locale.gen file (previously none of the lines had been uncommented), then running locale-gen.

Following this link solves my problem:

sudo localectl set-locale LANG=en_CA.UTF-8
# or change to en_US.UTF-8 depends on your locale-gen

it generates a file /etc/locale.conf that fixes this issue

I had this one as well. My solution was …

apt search locales

I got a list, then install this …

apt install locales-all