Using JSP variable in XSTL select

In a JSP I have the user language code (en, de, es, etc) stored in a variable. This value comes from the database.

In the other hand I have a XML file with all the values of the different languages, something like this:

<?xml version="1.0" encoding="UTF-8"?>
<language>
    <en enable="false" />
    <de enable="false" />
</emergency>

I am trying to use that variable in a XSTL select but nothing that I have tried works (and I tried a lot of crazy things):

<%@ taglib uri="http://ift.tt/Tb0L7s" prefix="c" %>
<%@ taglib uri="http://ift.tt/1gmwIFk" prefix="x" %>

<c:set var="USER_LANGUAGE" value="en" />

<c:import url="/languageData.xml" var="language" />
<x:parse xml="${language}" var="language" />

// work
// <x:if select="$language/en[@enabled = 'true']">

// don't work :(
<x:if select="$language/${USER_LANGUAGE}[@enabled = 'true']">
    // magic
</x:if>

Looks like I'm missing something, any help will be very welcome.

Thanks!

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