How to use a XSL inside an XML in Symphony CMS

I have a section called Products where every product is defined by Text Input and Multilingual Text Box. I have created over 100 products with custom description. At some point I need to place current year in Multilingual Text Box:

Lorem ipsum dolor <xsl:value-of select="$this-year" /> ipsum <a href="{$root}">Link to root</a>

which gives:

'Long Description' contains invalid XML. The following error was returned: loadXML(): Namespace prefix xsl on value-of is not defined in Entity

or I want to print data from Data Source:

Lorem ipsum
<xsl:variable name="products" select="/data/products" />
<xsl:for-each select="$products">
    //... do other XSL stuff in XML
</xsl:for-each>

which of course will cause error too.

Please take in account that I am total beginner in Symphony/XSLT and some of conceptions are still not well understood by me.