PostgreSQL 8.4: How to tell if a procedural language is installed or not?

Question

I have a setup program that requires plpgsql to install stored procedures in a PostgreSQL 8.4 database. I need to make sure the language is installed or the app will fail. I don't want to drop the language and re-add it, as that could screw some other things up.

Is there a way to install the language "gently"?
CREATE LANGUAGE IF NOT EXISTS does not appear to be valid.