substitute: Install the client's locale.

* guix/store.scm (set-build-options): Add #:locale parameter and honor
it.
* guix/scripts/substitute.scm (guix-substitute): Install the client's
locale.

guix/scripts/substitute.scm

@@ -1000,6 +1000,13 @@ default value."
   (newline)
   (force-output (current-output-port))
 
+  ;; Attempt to install the client's locale, mostly so that messages are
+  ;; suitably translated.
+  (match (or (find-daemon-option "untrusted-locale")
+             (find-daemon-option "locale"))
+    (#f     #f)
+    (locale (false-if-exception (setlocale LC_ALL locale))))
+
   (with-networking
    (with-error-handling                           ; for signature errors
      (match args

guix/store.scm

@@ -534,7 +534,10 @@ encoding conversion errors."
                             (substitute-urls #f)
 
                             ;; Number of columns in the client's terminal.
-                            (terminal-columns (terminal-columns)))
+                            (terminal-columns (terminal-columns))
+
+                            ;; Locale of the client.
+                            (locale (false-if-exception (setlocale LC_ALL))))
   ;; Must be called after `open-connection'.
 
   (define socket
@@ -573,6 +576,9 @@ encoding conversion errors."
                      ,@(if terminal-columns
                            `(("terminal-columns"
                               . ,(number->string terminal-columns)))
+                           '())
+                     ,@(if locale
+                           `(("locale" . ,locale))
                            '()))))
         (send (string-pairs pairs))))
     (let loop ((done? (process-stderr server)))