Hi @del65008 , I am assuming you are looking this for the XML Editor that comes with XML Add on or AEM Guides. You can refer - https://helpx.adobe.com/content/dam/help/en/xml-documentation-solution/4-1/Adobe-Experience-Manager-... to get detailed steps on how to customize AEM's default dictionary.
Just for info: Prior & up to AEM 6.4.0 that was XML Editor and the Add-on, from AEM 6.5.0 onwards, Add-on is placed under Tools that is - XML Documentation
Please check here for more details: https://experienceleaguecommunities.adobe.com/t5/adobe-experience-manager/how-to-enable-xml-editor-o...
While switching to AEM spell check in XML editor toolbar, Authors should be able to select or right-click a word and choose "Add to Dictionary". Authors need an interface from XML Editor to be able to add words to the dictionary