I don't remember whether this is a known bug or a design flaw, but you currently end up with the default system locale when using LightDM with automatic login on Arch.
My fix until today has been to configure my window manager,
awesome, using the
Lua-POSIX modules
to set the desired locale via rc.lua like this:
-- fix locale
pox = require("posix")
pox.setenv("LC_ALL", "de_DE.UTF-8")
pox.setenv("LANG", "de_DE.UTF-8")
When I switched to i3 yesterday I ended up with
the wrong locale again. Searching the net I stumbled upon
this article
which made me set up ~/.pam_environment to contain my locale:
LC_ALL=de_DE.UTF-8
LANG=de_DE.UTF-8
Guess what, works like a charm :)