Dynamically replacing substring in an given data(XML to fixed length) - xslt-2.0

Question

Actually I have started with my XSLT work recently, I am facing difficulty in solving one of the requirement.
I am trying to fetch an substring from DATA element in the mentioned input i,e is ECHO and OKAY these codes need to be replaced with the values present under CODE/ECHO and CODE/OKAY in the same input. I had tried storing the substring in a variable and as the variable value and tag value would be same, I have tried to fetch that in . But its not working.
Is it that we cant use variables in the XPATHS or there is some other representation which needs to be used? Could anyone please help me with this.
<?xml version="1.0" encoding="UTF-8"?>
<xsl:stylesheet version="2.0"
xmlns:xsl="http://www.w3.org/1999/XSL/Transform"
xmlns:fn="http://www.w3.org/2005/xpath-functions">
<xsl:output omit-xml-declaration="yes" />
<xsl:param name="break" select="'
'" />
<xsl:template match="/">
<xsl:variable name="String" select="substring(DATA, (string-length(substring(DATA,0,77)) + 1), 4)" />
<xsl:variable name="String1" >
<xsl:value-of select="Root/CODES/$String" />
</xsl:variable>
<xsl:value-of select="$break" />
<xsl:value-of select="$String1" />
</xsl:for-each>
</xsl:template>
Input:
<?xml version='1.0' encoding='utf-8'?>
<ROOT>
<INPUT>
<I_FILENAME>ERES</I_FILENAME>
</INPUT>
<CODES>
<ECHO>A1</ECHO>
<OKAY>A2</OKAY>
</CODES>
<TABLES>
<T_ER>
<item>
<DATA> HEADERERESRGCITIS220190301124112000000RGERSD46</DATA>
</item>
<item>
<DATA>000000 ABCD EF 0000000000 2018-11-060000000000EF 000000000000010000ECHO00400300000000000XXXXXX 000{ P 2018-11-05</DATA>
</item>
<item>
<DATA>000000 ABCD EF 0000000000 2018-11-060000000000EF 000000000000010000OKAY00400300000000000XXXXXX 000{ P 2018-11-05</DATA>
</item>
<item>
<DATA>TRAILERERESRGCITIS220190301124112000000001570000</DATA>
</item>
</T_ER>
</TABLES>
</ROOT>
EXPECTED OUT PUT:
HEADERERESRGCITIS220190301124112000000RGERSD46
000000 ABCD EF 0000000000 2018-11-060000000000EF 000000000000010000A100400300000000000XXXXXX 000{ P 2018-11-05
000000 ABCD EF 0000000000 2018-11-060000000000EF 000000000000010000A200400300000000000XXXXXX 000{ P 2018-11-05

Answer 1

<xsl:template match="INPUT|CODES">
</xsl:template>
<xsl:template match="TABLES">
<xsl:variable name="break" select="'
'" />
<xsl:for-each select="T_ER/item"><xsl:value-of select="$break"></xsl:value-of>
<xsl:value-of select="DATA"/>
</xsl:for-each>
</xsl:template>