A simple and straightforward way to manage message translations in Google Apps Script projects

Traducción.gs

// This object stores text strings in several languages inside sub-objects {ES}, {EN_US}...
const TRANSLATIONS = {
  ES: {
    MSG_HELLO: 'Hola',
    MSG_GOODBYE: 'Adiós'
  },
  EN_US: {
    MSG_HELLO: 'Hello',
    MSG_GOODBYE: 'Goodbye'
  }
  // More translations...
};

// Session.getActiveUserLocale() is not working properly at this moment in the new IDE
// https://issuetracker.google.com/issues/179563675
function showMessages(local = Session.getActiveUserLocale()) {
  // translation will reference the proper sub-object according to the selected or auto-detected locale
  let translation;
  switch(local) {
    case 'es':
      translation = TRANSLATIONS.ES;
      break;
    case 'en_us':
      translation = TRANSLATIONS.EN_US;
      break;
    /* Other locales here */
    default:
      translation = TRANSLATIONS.EN_US;
  }

  // Output messages
  console.info(translation.MSG_HELLO, translation.MSG_GOODBYE);
}

function test() {
  showMessages('es');
}