diff --git a/reference/forms/types/language.rst b/reference/forms/types/language.rst
index cdd5e461ed4..64d656c46dd 100644
--- a/reference/forms/types/language.rst
+++ b/reference/forms/types/language.rst
@@ -13,7 +13,8 @@ in the `International Components for Unicode`_ (e.g. ``fr`` or ``zh_Hant``).
 
 .. note::
 
-   The locale of your user is guessed using :phpmethod:`Locale::getDefault`
+   The locale of your user is guessed using :phpmethod:`Locale::getDefault`,
+   which requires to have installed and enabled the ``intl`` PHP extension.
 
 Unlike the ``ChoiceType``, you don't need to specify a ``choices`` option as the
 field type automatically uses a large list of languages. You *can* specify the option