message: [Improvement] Locale: add warning when not correctly registered
diff: @@ -6,8 +6,13 @@ export default {
  $t() {
  const { name } = this.$options;
  const prefix = name ? camelize(name) + '.' : '';
  - const messages = this.$vantMessages[this.$vantLang];
  + if (process.env.NODE_ENV !== 'production' && !this.$vantMessages) {
  + console.warn('[Vant] Locale not correctly registered.');
  + return () => '';
  + }
  +
  + const messages = this.$vantMessages[this.$vantLang];
  return (path, ...args) => {
  const message = get(messages, prefix + path) || get(messages, path);
  return typeof message === 'function' ? message.apply(null, args) : message;
  