How can I do string operations in XSLT?

<xsl:when test="StatusData/Status/Temperature > 27">
        <td bgcolor="#ffaaaa">
          <xsl:value-of select="StatusData/Status/Temperature" />
        </td>
      </xsl:when>

test="translate(StatusData/Status/Temperature, ",", ".") > 27"

test="number(translate(StatusData/Status/Temperature, ",", ".")) > 27.0"