language: Also show UTF-8 locales which are not suffixed with .utf8

On at least Fedora, "locale -a" offers many locale variants which
differ only by codeset; for example both "en_US" (which uses
ISO-8859-1) and "en_US.utf8".

This suffixing code (which apparently long ago derives from
gdm-language.c), is hardcoded to suffix .utf8.  If those variants
*aren't* available (because the system just defaults to utf8, as is
the case with OpenEmbedded's eglibc), then we end up hiding the
languages.

But gnome-desktop has already validated all of the locales it returns
actually *are* UTF-8, via language_name_get_codeset_details().

Since we treat the list of languages from gnome-desktop as canonical,
we can just add both variants to the initial set.  This might cause
duplicated output for a system which happened to have both en_US
(UTF-8) and en_US.utf8.  But there's no reason to do that really, so
if someone does, we can tell them to just have en_US, and drop the
.utf8.

https://bugzilla.gnome.org/show_bug.cgi?id=709272