Retrieve user languages for all locale categories

g_get_language_names() only returns the language names for the
LC_MESSAGES category, so mixed locale scenarios would result in missing
languages. Now, the languages are listed for each individual category.

Note that this issue was only present with the user installation. For
the system installation, the locales were queried from localed, and all
categories were checked.

In order to work on GLib versions < 2.58, the code to get language
names for a category has been backported.

Fixes #3712.