Fallback code for g_date_time_format_locale() fetches translated
strings (such as day and month names) from .mo catalogues via
gettext. These strings always come in UTF-8 encoding, because
that is the encoding that glib sets when it initializes gettext
for itself.
However, the non-fallback code uses nl_langinfo() and expects
its results to be in locale-dependent encoding.

This mismatch can result in UTF-8 strings being converted to UTF-8,
producing gibberish.

Fix this by converting UTF-8 strings to locale-dependent encoding
before using them. Also fix the code that was already doing the locale->UTF-8
conversion to not convert the strings when they are already UTF-8-encoded.

https://bugzilla.gnome.org/show_bug.cgi?id=766092