added links to the locale description for ISO standards on ISO639-1 and ISO3166-1 Alpha-2

Author: DerStoffel

book/translation.rst

@@ -22,7 +22,7 @@ the user::
     The term *locale* refers roughly to the user's language and country. It
     can be any string that your application then uses to manage translations
     and other format differences (e.g. currency format). We recommended the
-    ISO639-1 *language* code, an underscore (``_``), then the ISO3166 *country*
+    `ISO639-1`_ *language* code, an underscore (``_``), then the `ISO3166 Alpha-2`_ *country*
     code (e.g. ``fr_FR`` for French/France).
 
 In this chapter, we'll learn how to prepare an application to support multiple
@@ -954,3 +954,5 @@ steps:
 .. _`strtr function`: http://www.php.net/manual/en/function.strtr.php
 .. _`ISO 31-11`: http://en.wikipedia.org/wiki/Interval_%28mathematics%29#The_ISO_notation
 .. _`Translatable Extension`: https://github.com/l3pp4rd/DoctrineExtensions
+.. _`ISO3166 Alpha-2`: http://en.wikipedia.org/wiki/ISO_3166-1#Current_codes
+.. _`ISO639-1`: http://en.wikipedia.org/wiki/List_of_ISO_639-1_codes