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ClassicEditor
  .create( editorElement, {
     plugins: [ ..., WProofreader],
     toolbar: [ ..., 'wproofreader'],
            wproofreader: {
                  lang: 'en_US', // sets the default language
                  serviceId: 'your-service-ID', // required for the Cloud version only
                  srcUrl: 'https://svc.webspellchecker.net/spellcheck31/wscbundle/wscbundle.js'
}
})