Unrecognized language plpgsql when CREATE FUNCTION?!

I have install all latest PostgreSQL 7.0.3 packages except python
into my Redhat Linux 6.2

When I CREATE FUNCTION fname() RETURN OPAQUE AS '
DECLARE
...;
BEGIN
....;
END;
' LANGUAGE 'plpgsql';

I got Unrecognized language plpgsql, it only recognized sql, C, internal
and
created_procedure_name.
Well, in Postgres documents has examples for Language plpgsql.
Please tell me what do I missing, what other packages I need to install?

Thank you very much!

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