Make `set-locale-environment' complete over locale names

* lisp/international/mule-cmds.el (get-locale-names): New function.
(set-locale-environment): Use it to allow completion (bug#9655).

etc/NEWS

@@ -3844,6 +3844,11 @@ locales.  They are also available as aliases 'ebcdic-cp-*' (e.g.,
 'cp278' for 'ibm278').  There are also new charsets 'ibm2xx' to
 support these coding-systems.
 
+---
+** New function 'get-locale-names'.
+This utility function returns a list of locale names on the current
+system.
+
 ---
 ** 'while-no-input-ignore-events' accepts more special events.
 The special events 'dbus-event' and 'file-notify' are now ignored in

lisp/international/mule-cmds.el

@@ -2610,6 +2610,31 @@ is returned.  Thus, for instance, if charset \"ISO8859-2\",
 (declare-function w32-get-console-codepage "w32proc.c" ())
 (declare-function w32-get-console-output-codepage "w32proc.c" ())
 
+(defun get-locale-names ()
+  "Return a list of locale names."
+  (cond
+   ;; On Windows we have a built-in method to get the names.
+   ((and (fboundp 'w32-get-locale-info)
+         (fboundp 'w32-get-valid-locale-ids))
+    (mapcar #'w32-get-locale-info (w32-get-valid-locale-ids)))
+   ;; Unix-ey hosts should have a command to output locales currently
+   ;; defined by the OS.
+   ((executable-find "locale")
+    (split-string (shell-command-to-string "locale -a")))
+   ;; Fall back on the list of all defined locales.
+   ((and locale-translation-file-name
+         (file-exists-p locale-translation-file-name))
+    (with-temp-buffer
+      (insert-file-contents locale-translation-file-name)
+      (let ((locales nil))
+        (while (not (eobp))
+          (unless (looking-at-p "#")
+            (push (cadr (split-string (buffer-substring
+                                       (point) (line-end-position))))
+                  locales))
+          (forward-line 1))
+        (nreverse locales))))))
+
 (defun locale-translate (locale)
   "Expand LOCALE according to `locale-translation-file-name', if possible.
 For example, translate \"swedish\" into \"sv_SE.ISO8859-1\"."
@@ -2650,8 +2675,8 @@ touch session-global parameters like the language environment.
 
 See also `locale-charset-language-names', `locale-language-names',
 `locale-preferred-coding-systems' and `locale-coding-system'."
-  (interactive "sSet environment for locale: ")
-
+  (interactive (list (completing-read "Set environment for locale: "
+                                      (get-locale-names))))
   ;; Do this at runtime for the sake of binaries possibly transported
   ;; to a system without X.
   (setq locale-translation-file-name