Expand the documentation of the language system in i18n

Problem/Motivation

This is a follow up to #2533254: LanguageInterface lacks @ingroup i18n.

Expand the documentation of the language system in the i18n documentation. Currently, there is only one reference to it.

Steps to reproduce

Proposed resolution

Remaining tasks

Write the documentation
Review
Commit

User interface changes

API changes

Data model changes

Release notes snippet