docs: add app.locale from Twig AppVariable

Author: SVillette

templates.rst

@@ -380,12 +380,18 @@ gives you access to these variables:
 ``app.current_route_parameters``
     An array with the parameters passed to the route of the current request or an
     empty array if no request is available (equivalent to ``app.request.attributes.get('_route_params')``)
+``app.locale``
+    The locale used in the current :ref:`locale switcher <locale-switcher>` context.
 
 .. versionadded:: 6.2
 
     The ``app.current_route`` and ``app.current_route_parameters`` variables
     were introduced in Symfony 6.2.
 
+.. versionadded:: 6.3
+
+    The ``app.locale`` variable was introduced in Symfony 6.3.
+
 In addition to the global ``app`` variable injected by Symfony, you can also
 inject variables automatically to all Twig templates as explained in the next
 section.

translation.rst

@@ -988,6 +988,8 @@ checks translation resources for several locales:
     add the missing translation to the log file. For details,
     see :ref:`reference-framework-translator-logging`.
 
+.. _locale-switcher:
+
 Switch Locale Programmatically
 ------------------------------