Procedural languages aren't built into Postgres. They are offered by loadable modules. Please refer to the documentation for the PL in question for details about the syntax and how the AS clause is interpreted by the PL handler.
There are currently three procedural languages available in the standard Postgres distribution (PLSQL, PLTCL and PLPERL), and other languages can be defined. Refer to Chapter 23 for more information.
Read the upstream documentation. For corrections, first check the current version.