totem-menu: Properly handle menu text for undetermined language track

As per get_language_name_no_und() code that is called when building the menu labels, we explicitly set generic track name if no valid language is set. However to check this we verify if the string returned by gst_tag_get_language_name() is set, but this value is now a translated string in recent gstreamer versions.

So to ensure that the returned language name is a valid one let's confirm it via calling gst_tag_check_language_code().