The LanguageApp service of Apps Script can be used to translate any selected piece of text in a Google Document from one language to another. Select a paragraph in the document, or a word or a partial sentence, and choose the translate option from the menu bar. It will append the translated text with the original selection.

The functionality is offered by existing add-ons but this will give you an idea of what happens behind the scenes.

function translateSelection() {
  var selection = DocumentApp.getActiveDocument().getSelection();

  if (!selection) {
    DocumentApp.getUi().alert('Please select text for translation.');
    return;
  }

  var elements = selection.getSelectedElements();

  /* Append the translated text with the original */
  for (var i = 0; i < elements.length; i++) {
    if (elements[i].isPartial()) {
      /* Is partial text selected */

      var element = elements[i].getElement().asText();
      var startIndex = elements[i].getStartOffset();
      var endIndex = elements[i].getEndOffsetInclusive();

      var text = element.getText().substring(startIndex, endIndex + 1);
      var translatedText = LanguageApp.translate(text, 'ja', 'en');
      element.insertText(endIndex + 1, ' [' + translatedText + '] ');
    } else {
      var element = elements[i].getElement();

      if (element.editAsText) {
        /* Is the selection contains text */
        var text = element.asText().getText();
        var translatedText = LanguageApp.translate(text, 'ja', 'en');
        element.asText().setText(text + ' [' + translatedText + '] ');
      }
    }
  }
}