minor symfony#8211 Update language.rst (Draeli, javiereguiluz)

This PR was submitted for the 3.2 branch but it was merged into the 2.7 branch instead (closes symfony#8211).

Discussion
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Update language.rst

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

Commits
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a7ee84f Reword
1d318a0 Update language.rst

Author: xabbuh

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 ``choice`` type, you don't need to specify a ``choices`` or
 ``choice_list`` option as the field type automatically uses a large list