Maybe these guides could help you:
http://en.miui.com/thread-10716-1-1.html
http://miui-germany.de/forum2/index.php?page=Thread&threadID=7110
you need to add the option for spanish language, "<item>es_ES</item>" , in -->framework-miui-res.apk/res/values and edit array.xml,
sorry for my...