Daniele,
The version of CKEditor used in 8.5.3 is 3.5(.x). I recently needed a feature in the 3.6 release (readOnly) so I downloaded the latest CKEditor version and stored it in the domino/html folder on my server. In that version I could easily activaty the spell check option (scayt). The code needed to load a custom CKeditor version and enable the spell check option can be found below.
I did a quick test using the embedded CKEditor release (adding scayt_autoStartup and changing the disableNativeSpellChecker parameters in the config.js file), but wasn't able to activate it.
<?xml version="1.0" encoding="UTF-8"?>
<xp:view xmlns:xp="http://www.ibm.com/xsp/core">
<xp:this.resources>
<xp:headTag tagName="script">
<xp:this.attributes>
<xp:parameter name="src" value="/ckeditor36/ckeditor.js" />
<xp:parameter name="type" value="text/javascript" />
</xp:this.attributes>
</xp:headTag>
</xp:this.resources>
<xp:this.data>
<xp:dominoDocument var="document1" formName="fEditorDemo"></xp:dominoDocument>
</xp:this.data>
<xp:inputTextarea id="inputTextarea1" value="#{document1.body}"></xp:inputTextarea>
<xp:scriptBlock id="scriptBlock2">
<xp:this.value><![CDATA[CKEDITOR.replace( '#{id:inputTextarea1}',
{
uiColor : '#E2EBF1',
toolbarCanCollapse : true,
toolbar :
[
{ name: 'basicstyles', items : [ 'Bold','Italic','Underline','Subscript','Superscript' ] },
{ name: 'styles', items : [ 'Format','Font','FontSize' ] },
{ name: 'paragraph', items : [ 'NumberedList','BulletedList','-','Outdent','Indent' ] },
{ name: 'paragraph', items : [ 'JustifyLeft','JustifyCenter','JustifyRight','JustifyBlock'] },
{ name: 'insert', items : [ 'Table','SpecialChar' ] },
{ name: 'document', items : [ 'Source' ] }
],
readOnly : false,
// Disables the built-in spell checker while typing natively available in the browser (currently Firefox and Safari only).
disableNativeSpellChecker : true,
// If enabled (true), turns on SCAYT (SpellCheckAsYouType) automatically after loading the editor.
scayt_autoStartup :true
}); ]]></xp:this.value>
</xp:scriptBlock>
<xp:button value="Save" id="button1"><xp:eventHandler event="onclick" submit="true" refreshMode="complete">
<xp:this.action>
<xp:save></xp:save>
</xp:this.action></xp:eventHandler>
</xp:button>
</xp:view>