mergeObject2.js

const a = {
  CoolButton: {
    fr: "Omelette du fromage",
    en: "Black pudding"
  },
  NotSoCoolButton: {
    fr: "Une baguette",
    en: "Apple pie"
  }
};

const b = {
  CoolButton: {
    "pt-br": "Feijoada"
  },
  NotSoCoolButton: {
    "fr": "Je suis un valeur modifié",
    "pt-br": "Frango frito"
  }
};

function isObject(item) {
  return item && typeof item === "object" && !Array.isArray(item);
}

function mergeDeep(target, source) {
  let output = Object.assign({}, target);
  if (isObject(target) && isObject(source)) {
    Object.keys(source).forEach(key => {
      if (isObject(source[key])) {
        if (!(key in target)) {
          Object.assign(output, { [key]: source[key] });
        } else {
          output[key] = mergeDeep(target[key], source[key]);
        }
      } else {
        Object.assign(output, { [key]: source[key] });
      }
    });
  }
  return output;
}

console.log(mergeDeep(a, b))