xsl:value-of select after evaluating the variable

Hi,
I have xml document like following...  
==========

<de>German Text</de>
<en>English Text</en>

==========
I have an variable "$lang" which can be "de" or "en".

Now I want to extract /de or /en
depending on variable $lang.

I am trying something like
 <xsl:value-of select="node($lang)"/>

But I cant get the result!!
Any pointer?

thanx,
Shashi