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CREATE OR REPLACE FUNCTION public.create_plpgsql_language ()

        RETURNS TEXT

        AS $$

            CREATE LANGUAGE plpgsql;

            SELECT 'language plpgsql created'::TEXT;

        $$

LANGUAGE 'sql';

SELECT CASE WHEN

              (SELECT true::BOOLEAN

                 FROM pg_language

                WHERE lanname='plpgsql')

            THEN

              (SELECT 'language already installed'::TEXT)

            ELSE

              (SELECT public.create_plpgsql_language())

            END;

DROP FUNCTION public.create_plpgsql_language ();

----- code -----

First i create a helper function which actually will install the language, if called. This step is needed because i cannot put the CREATE LANGUAGE statement into the CASE clause. The next statement looks into pg_languages if "plpgsql" is already installed - if yes, just a message is returned - if no, the previously created function is called. Last but not least the function is dropped, it's not needed anymore.

I can execute this code from my Makefile and expect to get no error message even if the language already exist.