Feature: #93651 - Provide list of available system locales

See Issue #93651

Description

Every language of a site requires at least one locale which is used to format times, dates, currencies and other locale-dependent values. Additional locales can be added as fallback locales (comma separated).

The site configuration form for site languages provides the available locales as a select field, enabling easy selection of a value, rather than typing in the expected one, which might or might not be available.

Impact

Providing a list of available locales makes it faster and less error prone to setup a site and its languages.