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

[Eli Zaretskii <eliz@gnu.org>]

> From: "Drew Adams" <drew.adams@oracle.com>
> Date: Sun, 2 Oct 2011 12:45:03 -0700
> 
> 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?

Nowhere a mere mortal can see it.  The OS maintains the list of
supported locales, and will divulge it via a certain C-level API.  But
you cannot find that list by searching files or directories, AFAIK.
As an interactive user, you can see the list in the Control Panel, but
programmatically the list is available only through that API.

> 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?

Maybe, but I'm not sure a simple test for file's existence is worth
ugly system-dependent conditions.  If nothing else, it allows Windows
users to have locale translations, something that could be considered
a feature.