Recommendation for xml:lang expressions

In a VXML script, I prompt the caller for language and get a result from a <field>.  I need a way to programmatically express to all additional <audio> tags in my application that the language selected by my caller (let's say 'es') is to be used for TTS.  The <s xml:lang='es'> works if I wanted to hand-code this stuff and have <if> blocks all over the place to see what language to play the audio in, but I'm too lazy to do this much work.  I would really like to have a new attribute in the <s>, <p> and <voice> tags that lets me pass an ecmascript expression that is evaluated and placed into the xml:lang property before sending it over to the SSML engine.

As it is right now, in order to read a dollar amount in either Spanish or English, depending on what the caller chose, I have to do this:

<if cond="callerXmlLang == 'es'">
       <prompt>
              <s xml:lang="es">
                     <value expr="'$' + thisAmount"/>
              </s>
       </prompt>
</if>
<if cond="callerXmlLang == 'en'">
       <prompt>
              <s xml:lang="en">
                     <value expr="'$' + thisAmount"/>
              </s>
       </prompt>
</if>

I also considered using javascript to change the xml:lang attribute on the <vxml> tag, but this would be painful to have to set within every <form> that is called via <subdialog>, since the application context is fresh everytime you use <subdialog>.

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