use string equality in lang checks

`exists()` will not work as the variable already makes use of `string-join()` (to account for languages with multiple descriptions)

src/rp-schematron-base.sch

@@ -1454,11 +1454,11 @@
         <let name="xml-lang-value" value="@xml:lang"/>
         <let name="languages" value="'languages.xml'"/>
         <let name="subtag-description" value="string-join(document($languages)//*:item[@subtag=$xml-lang-value]/*:description,' / ')"/>
-        <assert test="exists($subtag-description)" 
+        <assert test="$subtag-description!=''" 
           role="error" 
           id="abstract-lang-test-1">The xml:lang attribute on <name/> must contain one of the IETF RFC 5646 subtags. '<value-of select="@xml:lang"/>' is not one of these values.</assert>
 
-        <report test="exists($subtag-description)" 
+        <report test="$subtag-description!=''" 
           role="warning" 
           id="abstract-lang-test-2"><name/> has an xml:lang attribute with the value '<value-of select="$xml-lang-value"/>', which corresponds to the following language: <value-of select="$subtag-description"/>. Please check this is correct.</report>
       </rule>

src/rp-schematron.sch

@@ -794,9 +794,9 @@
         <let name="xml-lang-value" value="@xml:lang"/>
         <let name="languages" value="'languages.xml'"/>
         <let name="subtag-description" value="string-join(document($languages)//*:item[@subtag=$xml-lang-value]/*:description,' / ')"/>
-        <assert test="exists($subtag-description)" role="error" id="abstract-lang-test-1">[abstract-lang-test-1] The xml:lang attribute on <name/> must contain one of the IETF RFC 5646 subtags. '<value-of select="@xml:lang"/>' is not one of these values.</assert>
+        <assert test="$subtag-description!=''" role="error" id="abstract-lang-test-1">[abstract-lang-test-1] The xml:lang attribute on <name/> must contain one of the IETF RFC 5646 subtags. '<value-of select="@xml:lang"/>' is not one of these values.</assert>
 
-        <report test="exists($subtag-description)" role="warning" id="abstract-lang-test-2">[abstract-lang-test-2] <name/> has an xml:lang attribute with the value '<value-of select="$xml-lang-value"/>', which corresponds to the following language: <value-of select="$subtag-description"/>. Please check this is correct.</report>
+        <report test="$subtag-description!=''" role="warning" id="abstract-lang-test-2">[abstract-lang-test-2] <name/> has an xml:lang attribute with the value '<value-of select="$xml-lang-value"/>', which corresponds to the following language: <value-of select="$subtag-description"/>. Please check this is correct.</report>
       </rule></pattern>
 
     <pattern id="front-permissions-tests-pattern"><rule context="front[journal-meta/lower-case(journal-id[1])='elife']//permissions" id="front-permissions-tests">

src/rp-schematron.xsl

@@ -5600,9 +5600,9 @@
       <xsl:variable name="subtag-description" select="string-join(document($languages)//*:item[@subtag=$xml-lang-value]/*:description,' / ')"/>
       <!--ASSERT error-->
       <xsl:choose>
-         <xsl:when test="exists($subtag-description)"/>
+         <xsl:when test="$subtag-description!=''"/>
          <xsl:otherwise>
-            <svrl:failed-assert xmlns:svrl="http://purl.oclc.org/dsdl/svrl" test="exists($subtag-description)">
+            <svrl:failed-assert xmlns:svrl="http://purl.oclc.org/dsdl/svrl" test="$subtag-description!=''">
                <xsl:attribute name="id">abstract-lang-test-1</xsl:attribute>
                <xsl:attribute name="role">error</xsl:attribute>
                <xsl:attribute name="location">
@@ -5617,8 +5617,8 @@
          </xsl:otherwise>
       </xsl:choose>
       <!--REPORT warning-->
-      <xsl:if test="exists($subtag-description)">
-         <svrl:successful-report xmlns:svrl="http://purl.oclc.org/dsdl/svrl" test="exists($subtag-description)">
+      <xsl:if test="$subtag-description!=''">
+         <svrl:successful-report xmlns:svrl="http://purl.oclc.org/dsdl/svrl" test="$subtag-description!=''">
             <xsl:attribute name="id">abstract-lang-test-2</xsl:attribute>
             <xsl:attribute name="role">warning</xsl:attribute>
             <xsl:attribute name="location">

test/tests/rp/abstract-lang-checks/abstract-lang-test-1/abstract-lang-test-1.sch

@@ -149,7 +149,7 @@
       <let name="xml-lang-value" value="@xml:lang"/>
       <let name="languages" value="'../../../../../src/languages.xml'"/>
       <let name="subtag-description" value="string-join(document($languages)//*:item[@subtag=$xml-lang-value]/*:description,' / ')"/>
-      <assert test="exists($subtag-description)" role="error" id="abstract-lang-test-1">[abstract-lang-test-1] The xml:lang attribute on <name/> must contain one of the IETF RFC 5646 subtags. '<value-of select="@xml:lang"/>' is not one of these values.</assert>
+      <assert test="$subtag-description!=''" role="error" id="abstract-lang-test-1">[abstract-lang-test-1] The xml:lang attribute on <name/> must contain one of the IETF RFC 5646 subtags. '<value-of select="@xml:lang"/>' is not one of these values.</assert>
     </rule>
   </pattern>
   <pattern id="root-pattern">

test/tests/rp/abstract-lang-checks/abstract-lang-test-2/abstract-lang-test-2.sch

@@ -149,7 +149,7 @@
       <let name="xml-lang-value" value="@xml:lang"/>
       <let name="languages" value="'../../../../../src/languages.xml'"/>
       <let name="subtag-description" value="string-join(document($languages)//*:item[@subtag=$xml-lang-value]/*:description,' / ')"/>
-      <report test="exists($subtag-description)" role="warning" id="abstract-lang-test-2">[abstract-lang-test-2] <name/> has an xml:lang attribute with the value '<value-of select="$xml-lang-value"/>', which corresponds to the following language: <value-of select="$subtag-description"/>. Please check this is correct.</report>
+      <report test="$subtag-description!=''" role="warning" id="abstract-lang-test-2">[abstract-lang-test-2] <name/> has an xml:lang attribute with the value '<value-of select="$xml-lang-value"/>', which corresponds to the following language: <value-of select="$subtag-description"/>. Please check this is correct.</report>
     </rule>
   </pattern>
   <pattern id="root-pattern">

test/xspec/rp-schematron.sch

@@ -1079,9 +1079,9 @@
         <let name="xml-lang-value" value="@xml:lang"/>
         <let name="languages" value="'languages.xml'"/>
         <let name="subtag-description" value="string-join(document($languages)//*:item[@subtag=$xml-lang-value]/*:description,' / ')"/>
-        <assert test="exists($subtag-description)" role="error" id="abstract-lang-test-1">The xml:lang attribute on <name/> must contain one of the IETF RFC 5646 subtags. '<value-of select="@xml:lang"/>' is not one of these values.</assert>
+        <assert test="$subtag-description!=''" role="error" id="abstract-lang-test-1">The xml:lang attribute on <name/> must contain one of the IETF RFC 5646 subtags. '<value-of select="@xml:lang"/>' is not one of these values.</assert>
 
-        <report test="exists($subtag-description)" role="warning" id="abstract-lang-test-2">
+        <report test="$subtag-description!=''" role="warning" id="abstract-lang-test-2">
         <name/> has an xml:lang attribute with the value '<value-of select="$xml-lang-value"/>', which corresponds to the following language: <value-of select="$subtag-description"/>. Please check this is correct.</report>
       </rule>
   </pattern>