userFormat.js

var lang = 'fr';
var timezone = 'Europe/Paris'; // Use timezone = null to use the browser timezone

moment.prototype.userFormat = function (format) {
  if (timezone) {
    this.tz(timezone);
  }

  if (lang) {
    this.locale(lang);
  }

  if (format) {
    return this.format(format);
  }

  return this.calendar();
};

console.log(
  moment('2018-05-12T23:16:46.123456Z').userFormat()
);

console.log(
  moment('2018-05-12T23:16:46.123456Z').userFormat('LLLL')
);

console.log(
  moment('2018-05-12T23:16:46.123456Z').userFormat('dd Do YY')
);