Error when calling Extension Function...

Hi,
When i am trying to call the xalan Extension  
Function node-set() to convert a String to a Node Set,
It is Giving me the following Error

file:/C:/xsltest/books.xsl; Line 18; Column -1; 
** Transformation error
   For extension function, could not find method
java.lang.String.node-set([ExpressionContext,] ).

My Code is as Follows
<xsl:param name="langID" select="'12'"/>
<xsl:variable name="lang" 
	 select="document(xalan:node-set($langID))">
</xsl:variable>

Or is there any Other Method to do This?

Thanks a Lot for All The Help
Ramesh


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