HASH256
</r:FileInfo Language="nl">
should look like </r:FileInfo>
In any case you should be looking at JSR223 PostProcessor and Groovy language so you would be able to:
<Legal>
tag valueExample code:
def data = new groovy.xml.XmlSlurper().parseText(prev.getResponseDataAsString())
def firstLegal = new groovy.xml.StreamingMarkupBuilder().bindNode(data.FileInfo.cSecurityNumbers.'*'[0]) as String
def hash256 = org.apache.commons.codec.digest.DigestUtils.sha256Hex(firstLegal)
Demo:
More information:
I cannot guarantee the above code will work as you expect due to the aforementioned 2 points, but it should guide you to the proper direction