user-accounts: fix language chooser response

If the language chooser is confirmed and no language is selected,
"Other..." is shown as user's language and the following error is printed:

(gnome-control-center:14608): AccountsService-CRITICAL **:
act_user_set_language: assertion 'language != NULL' failed
user-accounts: preset language chooser if language is known

Preset the language chooser similary as it is done in the region panel.
Also remove one goto command and share code.

https://bugzilla.gnome.org/show_bug.cgi?id=738002
user-accounts: fix language chooser response if language is unknown

If language is unknown and language chooser is cancelled, panel crash
with the following error:

(gnome-control-center:24952): Gtk-CRITICAL **: gtk_list_store_get_path:
assertion 'iter->stamp == priv->stamp' failed

(gnome-control-center:24952): Gtk-CRITICAL **: gtk_list_store_get_value:
assertion 'iter_is_valid (iter, list_store)' failed

(gnome-control-center:24952): GLib-GObject-WARNING **: gtype.c:4221:
type id '0' is invalid

(gnome-control-center:24952): GLib-GObject-WARNING **:
can't peek value table for type '<invalid>' which is not currently referenced
Segmentation fault (core dumped)

https://bugzilla.gnome.org/show_bug.cgi?id=738002