g_date_time_format: fix output in non-UTF-8 locales

In non-UTF-8 locales, the translations and nl_langinfo() return values
must be converted to UTF-8 before being returned to the caller.
Likewise, when making a recursive call to expand a format like '%x',
the format string must first be converted to UTF-8.

https://bugzilla.gnome.org/show_bug.cgi?id=668250