query: SELECT (concat(str((?decade-1)*10),"-",str((?decade)*10)) AS ?decade) (count(DISTINCT ?lang) as ?count)
  WHERE {
    ?lang wdt:P31/wdt:P279* wd:Q9143;
          wdt:P571 ?day.
    BIND( CEIL(year(?day)/10) as ?decade ).
  }
  GROUP by ?decade
  