As per Apple's documentation, a language ID is a code which identifies either a language used across multiple regions, a dialect from a specific region, or a script used in multiple regions. See the parse function for details.
When you parse a code with the forCode function, it will result in a table that contains a reference to the approprate cp.i18n.language table, and up to one of either the matching cp.i18n.region or cp.i18n.script tables. These contain the full details for each language/regin/script, as appropriate.
You can also convert the resulting table back to the code via tostring, or the code method.
Functions - API calls offered directly by the extension