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 two procedural languages available with the standard Postgres distribution (PLTCL and PLSQL), and other languages can be defined. Refer to Procedural Languages for more information.
Read the upstream documentation. For corrections, first check the current manual.