common: Backport g_get_language_names_with_category() more thoroughly

Even though we are going to check all the `LC_*` environment variables,
if one of them is not set, we still need to consider LANGUAGE, LC_ALL
and LANG; there is no guarantee that *every* `LC_*` environment variable
is set (and in particular, during our build-time tests, they will usually
not be).

Resolves: https://github.com/flatpak/flatpak/issues/4471


Signed-off-by: Simon McVittie <smcv@collabora.com>
(cherry picked from commit ae59fb7aca24cd2579aae51978f64bf762f78569)