invenio-recreate-demo-site: CFG_INVENIO_SITE_LANGS support

* Introduces support for CFG_INVENIO_SITE_LANGS variable
  to control the value stored in CFG_SITE_LANGS, which
  can greatly affect Invenio install speed.

Signed-off-by: Samuele Kaplun <samuele.kaplun@cern.ch>