Google Apps script to translate selections back and forth between English and Spanish

google-apps-translate.js

/**
 * @OnlyCurrentDoc Limits the script to only accessing the current sheet.
 */

function onOpen() {
  DocumentApp.getUi()
    .createMenu('Translate')
    .addItem('Translate selection to Spanish', 'translateSelectionToSpanish')
    .addItem('Translate selection to English', 'translateSelectionToEnglish')
    .addToUi();
}

function translateSelectionToSpanish() {
  translateSelection('en', 'es')
}

function translateSelectionToEnglish() {
  translateSelection('es', 'en')
}

function translateSelection(sourceLang, destLang) {
  const doc       = DocumentApp.getActiveDocument()
  const ui        = DocumentApp.getUi()
  const selection = doc.getSelection()

  if (!selection) {
    ui.alert('Nothing is selected')
  }

  const elements = selection.getRangeElements()
  const newSelection = doc.newRange()
  for (var i = 0; i < elements.length; i++) {
    const element = elements[i]

    // only modify elements that can be edited as text; skip images and other non-text elements.
    if (!element.getElement().editAsText) {
      return
    }
   
    // the selection can be within a text element, so we have to track the text, what came before it, and what came after it
    const textElement = element.getElement().editAsText()
    var preText = ''
    var text = textElement.getText()
    var postText = ''

    if (element.isPartial()) {
      preText  = text.substring(0, element.getStartOffset())
      postText = text.substring(1+element.getEndOffsetInclusive(), text.length)
      text     = text.substring(element.getStartOffset(), 1+element.getEndOffsetInclusive())
    }
    
    // now perform the translation
    const translated = LanguageApp.translate(text, sourceLang, destLang)
    
    // and update the contents
    textElement.setText(preText + translated + postText)
  
    // select the replaced text
    newSelection.addElement(textElement, preText.length, preText.length+translated.length-1)
  }
  
  doc.setSelection(newSelection.build())
}