Description:

From PostgreSQL documentation:

Postgres supports the definition of procedural languages. In the case
of a function or trigger procedure defined in a procedural language,
the database has no built-in knowledge about how to interpret the
function's source text. Instead, the task is passed to a handler that
knows the details of the language. The handler itself is a special
programming language function compiled into a shared object and loaded
on demand.

To enable PL/Tcl procedural language for your database you have to run
createlang command.