The pg_language system catalog table registers languages in which you can write functions or stored procedures. It is populated by CREATE LANGUAGE.

Table 1. pg_catalog.pg_language

column type references description
lanname name   Name of the language.
lanispl boolean   This is false for internal languages (such as SQL) and true for user-defined languages. Currently, pg_dump still uses this to determine which languages need to be dumped, but this may be replaced by a different mechanism in the future.
lanpltrusted boolean   True if this is a trusted language, which means that it is believed not to grant access to anything outside the normal SQL execution environment. Only superusers may create functions in untrusted languages.
lanplcallfoid oid pg_proc.oid For noninternal languages this references the language handler, which is a special function that is responsible for executing all functions that are written in the particular language.
lanvalidator oid pg_proc.oid This references a language validator function that is responsible for checking the syntax and validity of new functions when they are created. Zero if no validator is provided.
lanacl aclitem[]   Access privileges for the language.