XSLT is written in a very functional style, and in this style there is no equivalent of a break statement. What you can do is something like this:
<xsl:for-each select="...nodes...">
<xsl:if test="...some condition...">
...body of loop...
</xsl:if>
</xsl:for-each>
That way the for-each will still iterate through all the nodes, but the body of the loop will only be executed if the condition is true.