PostgreSQL includes several procedural languages with the base distribution: PL/pgSQL, PL/Tcl, PL/Perl, and PL/Python.
In addition, there are a number of procedural languages that are developed and maintained outside the core PostgreSQL distribution. A list of procedural languages is maintained on the PostgreSQL wiki. Note that some of these projects are not released under the same license as PostgreSQL. For more information on each procedural language, including licensing information, refer to its website and documentation.
Read the upstream documentation. For corrections, first check the current version.