bug#9655: 24.0.50; `set-locale-environment' should use completion

[Drew Adams]

The doc string says "LOCALE-NAME should be a string which is the name of
a locale supported by the system."  Isn't the set of locales supported
by the system something that Emacs can determine, and then make these
available as completion candidates?  Why should a user have to
investigate or guess which locales are choosable?
 
It also says: "The locale names supported by your system can typically
be found in a directory named `/usr/share/locale' or `/usr/lib/locale'."
This does not help users who are not on a UNIX/Gnu/Linux platform.
Where can they be found on Windows, for instance?
 
Some of the code looks as if it is specific to UNIX/Gnu/Linux/X-Window.
Should the setting of `locale-translation-file-name' be specific to such
systems, for instance?
 
It also looks like the code for this command should be split
up/factored.  Much of the command body is apparently about
non-interactive use (e.g. nil LOCALE-NAME) - factor that out, for
instance.
 

In GNU Emacs 24.0.50.1 (i386-mingw-nt5.1.2600)
 of 2011-09-19 on 3249CTO
Windowing system distributor `Microsoft Corp.', version 5.1.2600
configured using `configure --with-gcc (4.5) --no-opt'
 
Important settings:
  value of $LC_ALL: nil
  value of $LC_COLLATE: nil
  value of $LC_CTYPE: nil
  value of $LC_MESSAGES: nil
  value of $LC_MONETARY: nil
  value of $LC_NUMERIC: nil
  value of $LC_TIME: nil
  value of $LANG: ENU
  value of $XMODIFIERS: nil
  locale-coding-system: cp1252
  default enable-multibyte-characters: t