languages.rdf: Old English dialects (private use)

This change adds support for the regional dialects of Old English:

  *  ang            -- West Saxon     (Southern England)
  *  ang-x-kentish  -- Kentish        (South-Eastern England - Kent)
  *  ang-x-mercian  -- Mercian        (Central England)
  *  ang-x-northhum -- North-Humbrian (Northern England)

NOTE: West Saxon is used as the reference dialect of Old English
in phonology studies, so does not have an assigned private use tag.