Procedural languages aren't built into the PostgreSQL server; they are offered by loadable modules. Please refer to the documentation of the procedural language in question for details about the syntax and how the function body is interpreted for each language.
There are currently four procedural languages available in the standard PostgreSQL distribution: PL/pgSQL, PL/Tcl, PL/Perl, and PL/Python. Refer to Chapter 36 for more information. Other languages can be defined by users. The basics of developing a new procedural language are covered in Chapter 47.