installer: Run 'guix system init' with the right locale.

* gnu/installer/utils.scm (run-shell-command): Add #:locale and honor it.
* gnu/installer/newt/final.scm (run-install-shell): Add 'locale'
parameter; pass it to 'install-system'.
(run-final-page): Obtain locale from RESULT; pass it to 'run-install-shell'.
* gnu/installer/final.scm (install-system): Add 'locale' parameter; pass
it to 'run-shell-command'.

gnu/installer/final.scm

@@ -24,13 +24,15 @@
   #:use-module (guix build utils)
   #:export (install-system))
 
-(define (install-system)
+(define (install-system locale)
   "Start COW-STORE service on target directory and launch guix install command
-in a subshell."
+in a subshell.  LOCALE must be the locale name under which that command will
+run, or #f."
   (let ((install-command
          (format #f "guix system init ~a ~a"
                  (%installer-configuration-file)
                  (%installer-target-dir))))
     (mkdir-p (%installer-target-dir))
     (start-service 'cow-store (list (%installer-target-dir)))
-    (false-if-exception (run-shell-command install-command))))
+    (false-if-exception (run-shell-command install-command
+                                           #:locale locale))))

gnu/installer/newt/final.scm

@@ -65,22 +65,23 @@ press the button to reboot.")))
    (G_ "The final system installation step failed.  You can retry the \
 last step, or restart the installer.")))
 
-(define (run-install-shell)
+(define (run-install-shell locale)
   (clear-screen)
   (newt-suspend)
-  (let ((install-ok? (install-system)))
+  (let ((install-ok? (install-system locale)))
     (newt-resume)
     install-ok?))
 
 (define (run-final-page result prev-steps)
   (let* ((configuration   (format-configuration prev-steps result))
          (user-partitions (result-step result 'partition))
+         (locale          (result-step result 'locale))
          (install-ok?
           (with-mounted-partitions
            user-partitions
            (configuration->file configuration)
            (run-config-display-page)
-           (run-install-shell))))
+           (run-install-shell locale))))
     (if install-ok?
         (run-install-success-page)
         (run-install-failed-page))))

gnu/installer/utils.scm

@@ -54,9 +54,21 @@ number. If no percentage is found, return #f"
     (and result
          (string->number (match:substring result 1)))))
 
-(define (run-shell-command command)
+(define* (run-shell-command command #:key locale)
+  "Run COMMAND, a string, with Bash, and in the given LOCALE."
   (call-with-temporary-output-file
    (lambda (file port)
+     (when locale
+       (let ((supported? (false-if-exception
+                          (setlocale LC_ALL locale))))
+         ;; If LOCALE is not supported, then set LANGUAGE, which might at
+         ;; least give us translated messages.
+         (if supported?
+             (format port "export LC_ALL=\"~a\"~%" locale)
+             (format port "export LANGUAGE=\"~a\"~%"
+                     (string-take locale
+                                  (string-index locale #\_))))))
+
      (format port "~a~%" command)
      ;; (format port "exit~%")
      (close port)