Well there is XSLT 2.0 since 2007 (implemented by XSLT processors like Saxon 9, AltovaXML, XmlPrime) where you can simply do (assuming you have the declaration xmlns:xs="http://www.w3.org/2001/XMLSchema" on your xsl:stylesheet element):
<xsl:variable name="min" select="min(Rows/Entry/(@value1, @value2)/xs:decimal(translate(., ',', ''))"/>
<xsl:variable name="max" select="max(Rows/Entry/(@value1, @value2)/xs:decimal(translate(., ',', ''))"/>
If you really want to store a formatted string in a variable you can of course do that as well with e.g.
<xsl:variable name="min" select="format-number(min(Rows/Entry/(@value1, @value2)/xs:decimal(translate(., ',', '')), '#')"/>
<xsl:variable name="max" select="format-number(max(Rows/Entry/(@value1, @value2)/xs:decimal(translate(., ',', '')), '#')"/>
As for XSLT 1.0, there I think the sorting with for-each is the right approach but you would need to pull the xsl:variable outside the for-each e.g.
<xsl:variable name="min">
<xsl:for-each select="Rows/Entry/@value1|Rows/Entry/@value2">
<xsl:sort select="translate(., ',', '')" data-type="number"/>
<xsl:if test="position() = 1">
<xsl:value-of select="format-number(., '#')"/>
</xsl:if>
</xsl:for-each>
</xsl:variable>
<xsl:variable name="max">
<xsl:for-each select="Rows/Entry/@value1|Rows/Entry/@value2">
<xsl:sort select="translate(., ',', '')" data-type="number"/>
<xsl:if test="position() = last()">
<xsl:value-of select="format-number(.,'#')" />
</xsl:if>
</xsl:for-each>
</xsl:variable>
As an alternative you could replace the for-each with apply-templates and then write a template matching @value1 | @value2 but while I think most tasks to transform nodes are better done using push style in XSLT I think for finding a minimum or maximum value the for-each is fine.