message: add i18n watcher check
diff: @@ -2,7 +2,7 @@ import mixin from './mixin';
  import directive from './directive';
  import Config from './config';
  import Validator from './core/validator';
  -import { warn } from './core/utils';
  +import { warn, isCallable } from './core/utils';
  let Vue;
  @@ -20,7 +20,8 @@ function install (_Vue, options = {}) {
  Validator.localize(dictionary); // merge the dictionary.
  }
  - if (i18n) {
  + // try to watch locale changes.
  + if (i18n && i18n._vm && isCallable(i18n._vm.$watch)) {
  i18n._vm.$watch('locale', () => {
  Validator.regenerate();
  });
  