Update language.rst

Avoid taking lost time searching something which can be say clearly.

Author: Draeli

reference/forms/types/language.rst

@@ -14,6 +14,7 @@ in the `International Components for Unicode`_ (e.g. ``fr`` or ``zh_Hant``).
 .. note::
 
    The locale of your user is guessed using :phpmethod:`Locale::getDefault`
+   Don't forget install 'intl' package PHP to have translation.
 
 Unlike the ``choice`` type, you don't need to specify a ``choices`` or
 ``choice_list`` option as the field type automatically uses a large list