Note however, while plpgsql is installed into your database by default, plperl and plpython are not, so you would have to log into the database of interest and run the following (once), e.g. (only do for the language of interest, not all three; also note that depending on how you installed Postgres, you might need to also install extra packages):
Also note the difference between "trusted" and "untrusted" procedural languages. plpgsql and plperl are "trusted" while plperlu and plpythonu are "untrusted". See this link for an explanation: