I don't like the idea of yet another port with nothing than a binding
for setlocale(3), and I don't know which part of xmonad would benefit
from it.

Index: src/XMonad/Main.hs
--- src/XMonad/Main.hs.orig
+++ src/XMonad/Main.hs
@@ -15,7 +15,6 @@
 
 module XMonad.Main (xmonad, launch) where
 
-import System.Locale.SetLocale
 import qualified Control.Exception as E
 import Data.Bits
 import Data.List ((\\))
@@ -169,8 +168,6 @@ sendReplace = do
 -- your custom binary with the resume flag set to @True@.
 launch :: (LayoutClass l Window, Read (l Window)) => XConfig l -> Directories -> IO ()
 launch initxmc drs = do
-    -- setup locale information from environment
-    setLocale LC_ALL (Just "")
     -- ignore SIGPIPE and SIGCHLD
     installSignalHandlers
     -- First, wrap the layout in an existential, to keep things pretty: