[Backport 11.5] [TASK] Link supported languages page in TYPO3 Explain…

…ed (#399)

Instead of referencing to a PHP file, link to the existing page "Supported languages" in TYPO3 Explained.

Releases: main, 12.4, 11.5

Documentation/UserTsconfig/Setup.rst

@@ -134,8 +134,8 @@ lang
  language-key
 
 :aspect:`Description`
- One of the language-keys. For current options see
- :file:`typo3/sysext/core/Classes/Localization/Locales.php`, e.g. `dk`, `de`, `es` etc.
+ One of the language keys. For current options see
+ :ref:`t3coreapi:i18n_languages`, for example `dk`, `de`, `es` etc.
 
 
 .. index:: Records; Hide at copy