change moment.lang() to moment.locale()

As for moment ^2.8.0, the `moment.lang()` method is deprecated, so we use `locale()` method instead, note that the `localeData()` method refered in the deprecation message is not chainable, please see https://github.com/icambron/moment/commit/a45c1290b57eb9d7953f8bd71ab98736b943b9bc and https://github.com/moment/moment/pull/1819