diff --git a/guix/scripts/substitute.scm b/guix/scripts/substitute.scm
index f9d19fd735f0007e1686f69b134ff77d4389c8c2..117d8244495fb1bcaa188d74faed94a47446fa77 100755
--- a/guix/scripts/substitute.scm
+++ b/guix/scripts/substitute.scm
@@ -1126,12 +1126,13 @@ (define print-build-trace?
   ;; Sanity-check SUBSTITUTE-URLS so we can provide a meaningful error message.
   (for-each validate-uri (substitute-urls))
 
-  ;; Attempt to install the client's locale, mostly so that messages are
-  ;; suitably translated.
+  ;; Attempt to install the client's locale so that messages are suitably
+  ;; translated.  LC_CTYPE must be a UTF-8 locale; it's the case by default so
+  ;; don't change it.
   (match (or (find-daemon-option "untrusted-locale")
              (find-daemon-option "locale"))
     (#f     #f)
-    (locale (false-if-exception (setlocale LC_ALL locale))))
+    (locale (false-if-exception (setlocale LC_MESSAGES locale))))
 
   (catch 'system-error
     (lambda ()