[Form] Documented the choice_self_translation option of LanguageType

Author: javiereguiluz

reference/forms/types/language.rst

@@ -24,6 +24,7 @@ manually, but then you should just use the ``ChoiceType`` directly.
 | Rendered as | can be various tags (see :ref:`forms-reference-choice-tags`)           |
 +-------------+------------------------------------------------------------------------+
 | Options     | - `alpha3`_                                                            |
+|             | - `choice_self_translation`_                                           |
 |             | - `choice_translation_locale`_                                         |
 +-------------+------------------------------------------------------------------------+
 | Overridden  | - `choices`_                                                           |
@@ -74,6 +75,19 @@ If this option is ``true``, the choice values use the `ISO 639-2 alpha-3`_
 three-letter codes (e.g. French = ``fra``) instead of the default
 `ISO 639-1 alpha-2`_ two-letter codes (e.g. French = ``fr``).
 
+choice_self_translation
+~~~~~~~~~~~~~~~~~~~~~~~
+
+**type**: ``boolean`` **default**: ``false``
+
+.. versionadded:: 5.1
+
+    The ``choice_self_translation`` option was introduced in Symfony 5.1.
+
+If this option is ``true``, each choice value is translated into its own
+language instead of translating all of them into the current application locale
+(e.g. ``[..., 'cs' => 'čeština', 'es' => 'español', ..., 'zh' => '中文']``).
+
 .. include:: /reference/forms/types/options/choice_translation_locale.rst.inc
 
 Overridden Options