Yes, we support PL/pgSQL. All databases you create should have this functionality enabled. You do not need to use the CREATE LANG function.
We will consider addition of other languages on a case-by-case basis. If you need further assistance, please open a support ticket with our Guru Crew on the Customer Portal at https://my.a2hosting.com.
For more information about PL/pgSQL, please visit http://www.postgresql.org/docs/8.3/static/plpgsql.html.