menu: Fix "Undetermined" appearing in menu items

Due to recent changes in gst-plugins-base, some languages without a 2 letter-code are now getting mapped to languages using gst_tag_get_language_name(). Which isn't a problem, except that we were relying on gst_tag_get_language_name("und") returning NULL.

Check for "und" manually so that track menus get named properly.

See https://gitlab.freedesktop.org/gstreamer/gst-plugins-base/-/merge_requests/785